A file is identified in a COBOL program by a filename which is a user-defined word. Outside the COBOL system it is identified by a filename which must be in an external, operating-system format. There are four components:
The length of the filename is limited to 261 characters. The length of the basename and extension together is 256 characters.
COBOL imposes no other limits, but in order for the operating system to handle the filename correctly, any operating system limits must be observed when the program is run.