In the 4th Dimension Language Reference and also in the 4D method editor, commands that directly return a value are always shown Capitalized. For example:

docRef:=Open document("")

4D commands that do NOT return a value directly are always shown in UPPER CASE. For example:

CLOSE DOCUMENT(docRef)

Of course, just because a command does not directly return a value as a
function call does not mean it cannot return information through its parameters. For example:

VOLUME LIST(volumes)

This command does not directly return a value, but it does return information in the parameter "volumes". Also, either type of command may set the value of system variables such as OK.