When assigning a name to a symbol not all of the characters can be used.
Only letters, numbers and the underscore (_) are allowed.
 

In the comments field, all characters allowed.

There are other restrictions:
- A symbol name can not be only digits. 
     Otherwise it could also be seen as an address of the PLC.

- A symbol may not begin with a digit.

- No spaces.

See the following examples: