:: is used on the HP 48 series and HP 49 series calculators as the opening mark for a "secondary object", which is closed with ;. Secondaries are usually used to group a number of commands and functions for a SystemRPL program, but can be used to hold any other objects together as well, like a list (which on the calculator is delimited by { and }). Lists and secondaries are both examples of composite objects, and thus share many commands on the calculator. The :: and ; are similar to { and } in C, Java, and quite a few other languages.