When should I use curly braces and when not for variables?

There are multiple places we use variables, In Content, Embed, Pre Code and Post Code. But when should we use curly braces to make them work?

When to use?

If you are using variables in Content or Embed then you have to use curly braces to make them work. Otherwise they will be treated as normal texts

Example

Hello {user.mention}!

When not to use?

If you are using any variables in the Pre Code or Post Code you don't have to use any curly braces at all. In fact using them will cause errors.

Example

add_roles(user, 3904234823402349234)