Explain that no struct can be created as transaction input other than…

… some exceptions.

book/src/concepts/what-is-a-transaction.md

@@ -24,11 +24,15 @@ Transactions consist of:
 
 - a sender - the [account](./what-is-an-account.md) that _signs_ the transaction;
 - a list (or a chain) of commands - the operations to be executed;
-- command inputs - the arguments for the commands: either `pure` - simple values like numbers or
+- command inputs - the arguments for the commands: either `pure` - simple* values like numbers or
   strings, or `object` - objects that the transaction will access;
 - a gas object - the `Coin` object used to pay for the transaction;
 - gas price and budget - the cost of the transaction;
 
+<sup>* Notice that custom structures defined in Move packages cannot be created and passed to
+MoveVM as transaction inputs. They always need to be instantiated/created inside MoveVM. There are
+only some exceptions to this such as: `ID` and `String`</sup>
+
 ## Commands
 
 Sui transactions may consist of multiple commands. Each command is a single built-in command (like

book/src/programmability/bcs.md

@@ -52,6 +52,8 @@ Structs encode similarly to simple types. Here is how to encode a struct using B
 {{#include ../../../packages/samples/sources/programmability/bcs.move:encode_struct}}
 ```
 
+Note that encoding a struct does not make much sense in interacting with the Move VM, as custom structs cannot be created this way to use in MoveVM. See also [Transaction](../concepts/what-is-a-transaction.html).
+
 ## Decoding
 
 Because BCS does not self-describe and Move is statically typed, decoding requires prior knowledge of the data type. The `sui::bcs` module provides various functions to assist with this process.