Custom field names must uniquely identify their custom fields and are used in a number of back-end operations to reference those fields.
Aside from making a field name, make sure that each field name meets the following conditions:
•A unique alphanumeric combination
•Unique within its specified category
•Begins with a letter
•Does not include spaces, punctuation, or special characters, that is, a field name must be created from A through Z, a through z, and 0 through 9
•Is no more than 30 characters long, unless it is of the type Involved, then it must be no more than 26 characters long
•Is not a reserved word in your database server. (There are dozens of such words; refer to your database documentation for a list.)