The dictionary is a text datablock, and therefore can be read in at start-up or with the Read command. It contains free format text, with a keyword at the beginning of a line. Lines starting with an unknown keyword are simply skipped. Some keywords are only relevant to some commands, e.g. the 'centre' keyword is only used by the Template building system only, while the 'rotamer' keywords are only used by the roatmer commands. It is therefore important that the number of lines in the header should match with the contents.
Some commands expect keyword entries to be present.
For example, the rotamer based commands in O expect rotamer keywords for each residue. The amino-acid rotamers in this file are the result of an analysis made by Gerard Kleywegt. The rotamer names provide an idea of their frequency. Perhaps, you don't like to use the low frequencey rotamers. Just copy over the standard file and edit it to your own specifications.