2 files changed, 105 insertions, 21 deletions
diff --git a/doc/manual/src/command-ref/nix-collect-garbage.md b/doc/manual/src/command-ref/nix-collect-garbage.md
index 51db5fc67..a679ceaf7 100644
--- a/doc/manual/src/command-ref/nix-collect-garbage.md
+++ b/doc/manual/src/command-ref/nix-collect-garbage.md
@@ -1,6 +1,6 @@
 # Name
 
-`nix-collect-garbage` - delete unreachable store paths
+`nix-collect-garbage` - delete unreachable [store objects]
 
 # Synopsis
 
@@ -8,17 +8,57 @@
 
 # Description
 
-The command `nix-collect-garbage` is mostly an alias of [`nix-store
---gc`](@docroot@/command-ref/nix-store/gc.md), that is, it deletes all
-unreachable paths in the Nix store to clean up your system. However,
-it provides two additional options: `-d` (`--delete-old`), which
-deletes all old generations of all profiles in `/nix/var/nix/profiles`
-by invoking `nix-env --delete-generations old` on all profiles (of
-course, this makes rollbacks to previous configurations impossible);
-and `--delete-older-than` *period*, where period is a value such as
-`30d`, which deletes all generations older than the specified number
-of days in all profiles in `/nix/var/nix/profiles` (except for the
-generations that were active at that point in time).
+The command `nix-collect-garbage` is mostly an alias of [`nix-store --gc`](@docroot@/command-ref/nix-store/gc.md).
+That is, it deletes all unreachable [store objects] in the Nix store to clean up your system.
+
+However, it provides two additional options,
+[`--delete-old`](#opt-delete-old) and [`--delete-older-than`](#opt-delete-older-than),
+which also delete old [profiles], allowing potentially more [store objects] to be deleted because profiles are also garbage collection roots.
+These options are the equivalent of running
+[`nix-env --delete-generations`](@docroot@/command-ref/nix-env/delete-generations.md)
+with various augments on multiple profiles,
+prior to running `nix-collect-garbage` (or just `nix-store --gc`) without any flags.
+
+> **Note**
+>
+> Deleting previous configurations makes rollbacks to them impossible.
+
+These flags should be used with care, because they potentially delete generations of profiles used by other users on the system.
+
+## Locations searched for profiles
+
+`nix-collect-garbage` cannot know about all profiles; that information doesn't exist.
+Instead, it looks in a few locations, and acts on all profiles it finds there:
+
+1. The default profile locations as specified in the [profiles] section of the manual.
+
+2. > **NOTE**
+   >
+   > Not stable; subject to change
+   >
+   > Do not rely on this functionality; it just exists for migration purposes and is may change in the future.
+   > These deprecated paths remain a private implementation detail of Nix.
+
+   `$NIX_STATE_DIR/profiles` and `$NIX_STATE_DIR/profiles/per-user`.
+
+   With the exception of `$NIX_STATE_DIR/profiles/per-user/root` and `$NIX_STATE_DIR/profiles/default`, these directories are no longer used by other commands.
+   `nix-collect-garbage` looks there anyways in order to clean up profiles from older versions of Nix.
+
+# Options
+
+These options are for deleting old [profiles] prior to deleting unreachable [store objects].
+
+- <span id="opt-delete-old">[`--delete-old`](#opt-delete-old)</span> / `-d`\
+  Delete all old generations of profiles.
+
+  This is the equivalent of invoking `nix-env --delete-generations old` on each found profile.
+
+- <span id="opt-delete-older-than">[`--delete-older-than`](#opt-delete-older-than)</span> *period*\
+  Delete all generations of profiles older than the specified amount (except for the generations that were active at that point in time).
+  *period* is a value such as `30d`, which would mean 30 days.
+
+  This is the equivalent of invoking [`nix-env --delete-generations <period>`](@docroot@/command-ref/nix-env/delete-generations.md#generations-days) on each found profile.
+  See the documentation of that command for additional information about the *period* argument.
 
 {{#include ./opt-common.md}}
 
@@ -32,3 +72,6 @@ generations of each profile, do
 ```console
 $ nix-collect-garbage -d
 ```
+
+[profiles]: @docroot@/command-ref/files/profiles.md
+[store objects]: @docroot@/glossary.md#gloss-store-object
diff --git a/doc/manual/src/command-ref/nix-env/delete-generations.md b/doc/manual/src/command-ref/nix-env/delete-generations.md
index 92cb7f0d9..d828a5b9e 100644
--- a/doc/manual/src/command-ref/nix-env/delete-generations.md
+++ b/doc/manual/src/command-ref/nix-env/delete-generations.md
@@ -9,14 +9,39 @@
 # Description
 
 This operation deletes the specified generations of the current profile.
-The generations can be a list of generation numbers, the special value
-`old` to delete all non-current generations, a value such as `30d` to
-delete all generations older than the specified number of days (except
-for the generation that was active at that point in time), or a value
-such as `+5` to keep the last `5` generations ignoring any newer than
-current, e.g., if `30` is the current generation `+5` will delete
-generation `25` and all older generations. Periodically deleting old
-generations is important to make garbage collection effective.
+
+*generations* can be a one of the following:
+
+- <span id="generations-list">`<number>...`</span>:\
+  A list of generation numbers, each one a separate command-line argument.
+
+  Delete exactly the profile generations given by their generation number.
+  Deleting the current generation is not allowed.
+
+- The special value <span id="generations-old">`old`</span>
+
+  Delete all generations older than the current one.
+
+- <span id="generations-days">`<days>d`</span>:\
+  The last *days* days
+
+  *Example*: `30d`
+
+  Delete all generations older than *days* days.
+  The generation that was active at that point in time is excluded, and will not be deleted.
+
+- <span id="generations-count">`+<count>`</span>:\
+  The last *count* generations up to the present
+
+  *Example*: `+5`
+
+  Keep the last *count* generations, along with any newer than current.
+
+Periodically deleting old generations is important to make garbage collection
+effective.
+The is because profiles are also garbage collection roots — any [store object] reachable from a profile is "alive" and ineligible for deletion.
+
+[store object]: @docroot@/glossary.md#gloss-store-object
 
 {{#include ./opt-common.md}}
 
@@ -28,19 +53,35 @@ generations is important to make garbage collection effective.
 
 # Examples
 
+## Delete explicit generation numbers
+
 ```console
 $ nix-env --delete-generations 3 4 8
 ```
 
+Delete the generations numbered 3, 4, and 8, so long as the current active generation is not any of those.
+
+## Keep most-recent by count count
+
 ```console
 $ nix-env --delete-generations +5
 ```
 
+Suppose `30` is the current generation, and we currently have generations numbered `20` through `32`.
+
+Then this command will delete generations `20` through `25` (`<= 30 - 5`),
+and keep generations `26` through `31` (`> 30 - 5`).
+
+## Keep most-recent in days
+
 ```console
 $ nix-env --delete-generations 30d
 ```
 
+This command will delete all generations older than 30 days, except for the generation that was active 30 days ago (if it currently exists).
+
+## Delete all older
+
 ```console
 $ nix-env --profile other_profile --delete-generations old
 ```
-