feat: explanation of remaining serialization format

pages/book/cells.mdx

@@ -103,8 +103,7 @@ While you may use them for [manual parsing](#cnp-manually) of the cells, it's st
 Similar to serialization options of [`Int{:tact}`](/book/integers) type, `Cell{:tact}`, `Builder{:tact}` and `Slice{:tact}` also have various representations for encoding their values in the following cases:
 
 * as fields of [contracts](/book/contracts) and [traits](/book/types#traits),
-* as fields of [Structs](/book/structs-and-messages#structs) and [Messages](/book/structs-and-messages#messages),
-* and as key/value types of [maps](/book/maps).
+* and as fields of [Structs](/book/structs-and-messages#structs) and [Messages](/book/structs-and-messages#messages).
 
 ```tact
 contract SerializationExample {
@@ -113,11 +112,47 @@ contract SerializationExample {
 }
 ```
 
-### `remaining` [#serialization-bytes64]
+### `remaining` [#serialization-remaining]
+
+By default, when it comes to storing values of [`Cell{:tact}`](#cells), [`Builder{:tact}`](#builders) and [`Slice{:tact}`](#slices) types in persistent storage of loading them from the received messages or assigned values, Tact's auto-layout algorithm uses references (refs):
+
+```tact
+contract ReReReF {
+    c: Cell;    // ^cell in TL-B notation
+    b: Builder; // ^builder in TL-B notation
+    s: Slice;   // ^slice in TL-B notation
+}
+```
+
+However, using `remaining{:tact}` makes values assigned to those fields to be stored directly as a [`Slice{:tact}`](#slices):
+
+```tact
+contract RemRemRem {
+    c: Cell as remaining; // remainder<cell> in TL-B notation,
+                          // storing and loading Slices directly in this Cell
+
+    b: Builder as remaining; // remainder<builder> in TL-B notation,
+                             // storing and loading Slices directly in this Builder
+
+    s: Slice as remaining; // remainder<slice> in TL-B notation,
+                           // storing and loading Slices directly
+}
+```
+
+That's like using [`Builder.storeSlice(){:tact}`][b-5] and [`Slice.loadSlice(){:tact}`][s-5] over [`Builder.storeRef(){:tact}`][b-8] and [`Slice.loadRef(){:tact}`][s-8], which are to be used by default. In practice, this makes using `Slice as remaining{:tact}` the most [gas](https://docs.ton.org/develop/smart-contracts/fees#gas)-efficient option for working with [`Slice{:tact}`](#slices) values, since it uses the least amount of operations and bits.
 
 <Callout>
 
-  To be resolved by [#26](https://github.com/tact-lang/tact-docs/issues/26).
+  Note, that the field serialized `as remaining{:tact}` cannot be [optional](/book/optional). That is, specifying something like `Cell? as remaining{:tact}` would cause a compilation error.
+
+  Also note, that as of now, specifying `remaining{:tact}` for `Cell{:tact}` as the [map](/book/maps) value type doesn't do anything — the cell would still be stored as a reference:
+
+  ```tact
+  struct Nope {
+      m: map<Int, Cell as remaining>; // dict<uint8, ^cell> in TL-B notation,
+                                      // despite the `remaining`
+  }
+  ```
 
 </Callout>