docs: a new dimension in an autopopulated table requires a part table

src/explanation/entity-integrity.md

@@ -239,7 +239,7 @@ this table?"*
 - Adding a dimension makes the grain **finer**: a part table that adds `blob_idx`
   (`Detection.Blob` below) has grain **(… × blob)** — one row per blob of a
   detection.
-- A computed table's grain is the combination of dimensions at which its `make()`
+- An autopopulated table's grain is the combination of dimensions at which its `make()`
   produces rows — its
   [`key_source`](../reference/specs/autopopulate.md). *A computation operates at
   its grain.*
@@ -305,11 +305,12 @@ class SubjectProfile(dj.Manual):
 one profile per subject. In schema diagrams, such tables have
 **non-underlined names**.
 
-### Computed tables never introduce dimensions
+### Autopopulated tables introduce dimensions only through part tables
 
-A `Computed` table's primary key is fully inherited from its dependencies.
-New entity types are introduced by Manual or Lookup tables, not by
-computation:
+An autopopulated table — `Computed` or `Imported` — runs its `make()` once per
+key in its [`key_source`](../reference/specs/autopopulate.md), so the master's
+primary key is fully inherited from its dependencies. The master introduces no
+dimension of its own:
 
 ```python
 @schema
@@ -322,10 +323,8 @@ class SessionSummary(dj.Computed):
     """
 ```
 
-### Part tables CAN introduce dimensions
-
-Unlike Computed master tables, part tables can introduce new dimensions
-when a single computation produces multiple related results:
+When one `make()` call produces several results that need their own identifier,
+that new dimension goes in a **part table**:
 
 ```python
 @schema
@@ -350,6 +349,12 @@ class Detection(dj.Computed):
 `Detection` inherits its dimensions; `Detection.Blob` introduces `blob_idx`
 to identify individual blobs within each detection.
 
+> **Introducing a new dimension in an autopopulated table requires a part
+> table.**
+
+The master row and its part rows are inserted together in one transaction, so
+the new dimension is populated atomically with the computation that defines it.
+
 ### Dimensions and attribute lineage
 
 Every foreign-key attribute traces back to the dimension where it was first

src/how-to/model-relationships.ipynb

@@ -1657,7 +1657,7 @@
    "source": [
     "**Thick solid line** to a **red (Computed) table** that is **not underlined**.\n",
     "\n",
-    "Computed tables never introduce dimensions — their primary key is entirely inherited from dependencies."
+    "An autopopulated master introduces no dimension of its own — its primary key is entirely inherited from dependencies. A new dimension in an autopopulated table requires a part table."
    ]
   },
   {

src/how-to/read-diagrams.ipynb

@@ -499,8 +499,8 @@
     "| Not underlined | Exists in the space defined by dimensions from referenced tables |\n",
     "\n",
     "**Key rules:**\n",
-    "- Computed tables **never** introduce dimensions (always non-underlined)\n",
-    "- Part tables **can** introduce dimensions (may be underlined)"
+    "- An autopopulated master (`Computed`, `Imported`) is always non-underlined — its primary key is fully inherited from its dependencies\n",
+    "- A part table **can** be underlined; a part table is the only way an autopopulated table introduces a new dimension"
    ]
   },
   {