An identifier is a sequence of letters, numbers and underlines beginning with a letter or underline (e.g., name of a function block type, an instance, a variable or a section). If you select the Extended option in the Tools > Project Settings... > Variables dialog, letters from national character sets (e.g., ö, ü, é, õ) can be used. Underlines are significant in identifiers; e.g., A_BCD and AB_CD are interpreted as different identifiers. Ending underlines is invalid.

Identifiers cannot contain spaces. Not case sensitive; e.g., ABCD and abcd are interpreted as the same identifier.

According to IEC 61131-3 leading digits are not allowed in identifiers. Nevertheless, you can use them if you activate the check box Allow leading digits in the Tools > Project Settings... > Variables dialog.

According to IEC 61131-3 multiple leading underlines and consecutives underlines are not allowed in identifiers. Nevertheless, you can use them if you select the Extended option in the Tools > Project Settings... > Variables > Character set dialog.

Identifiers cannot be keywords.