I will refer to them as ‘arguments’ versus ‘braced groups’.
If you’re not aware which construct is in force, then you can get unexpected results.
The examples below should clarify.
ARGUMENTS:
In this documentation, arguments are indicated by #1, #2, etc.
An argument is either a single ‘token’ (like ‘a’ or ‘\alpha’), or is a group enclosed in braces.
For example, the \boldsymbol command takes an argument, notated by: