User`s guide

How to Type a Command with Its Parameter(s)
Conventions Used to Describe File Specifications
You name or specify a file with a file specification, called a filespec. This consists
of a device name, the file name, and the file type in the following format. A colon
(:) separates the device name from the file name, and a period (.) separates the file
name from the file type.
dev:filnam.typ
where:
dev: is either a logical-device name or a physical-device name, which is
a one- to three-character name. You can omit this name if the file
is on your default storage device.
filnam is a one- to six-character alphanumeric name of the file.
.typ is a zero- to three-character alphanumeric file type.
You can omit the file type in some file specifications, if your file
has the default file type RT–11 assumes for the operation. For
example, the PRINT command assumes a LST file type and the
MACRO command assumes a MAC file type for input files. Many
commands, however, do not assume a file type.
How to Use DCL Commands 1–3