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

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

@@ -29,6 +29,27 @@ Transactions consist of:
 - a gas object - the `Coin` object used to pay for the transaction;
 - gas price and budget - the cost of the transaction;
 
+## Inputs
+
+Transaction inputs are the arguments for the transaction and are split between 2 types:
+- Pure arguments: These are mostly [primitive types](../move-basics/primitive-types.html) with some
+extra additions. A pure argument can be:
+    - boolean
+    - integer (u8, u16, u32, u64, u128, u256)
+    - address
+    - std::ascii::String
+    - std::string::String
+    - vector of pure types
+    - Option of pure types
+    - ID, a special type that refers to a unique identifier of an object
+- Object arguments: These are objects or references of objects that the transaction will access. An
+object argument needs to be either a shared object, a frozen object, or an object that the
+transaction sender owns, in order for the transaction to be successfull.
+For more see [Object Model](http://localhost:3000/object/index.html).
+
+Note that a single command inside a transaction can also have a previous command's result as input.
+These results/inputs can be anything from a simple value, an object, or any kind of Move struct.
+
 ## Commands
 
 Sui transactions may consist of multiple commands. Each command is a single built-in command (like

book/src/programmability/bcs.md

@@ -52,6 +52,10 @@ 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 custom struct does not make much sense in interacting with the Move VM, as
+they cannot be created this way to use in Move VM. See also
+[Transaction Inputs](../concepts/what-is-a-transaction.html#inputs).
+
 ## 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.