the regular rules for identifiers in mssql server are : for the first character :
The underscore (_), at sign (@), or number sign (#).
Certain symbols at the beginning of an identifier have special meaning in SQL Server. A regular identifier that starts with the at sign always denotes a local variable or parameter and cannot be used as the name of any other type of object. An identifier that starts with a number sign denotes a temporary table or procedure. An identifier that starts with double number signs (##) denotes a global temporary object. Although the number sign or double number sign characters can be used to begin the names of other types of objects, we do not recommend this practice.
Some Transact-SQL functions have names that start with double at signs (@@). To avoid confusion with these functions, you should not use names that start with @@.