Is there a name for the class of characters represented by [] in SQL Server?
"I think" this is what you are referring to:

Delimited Identifiers

https://technet.microsoft.com/en-us/library/ms176027(v=sql.105).aspx

T-SQL Naming Conventions

https://docs.microsoft.com/en-us/sql/t-sql/language-elements/transact-sql-syntax-conventions-transact-sql