Reserving licenses is an RLM feature and is documented in the RLM User Guide (see “The ISV Options File” section).

You use the RESERVE keyword in the ISV options file (solidangle.opt) to reserve licenses for specific users, machine names, or IP addresses.

For example, this reserves 2 licenses for machines with the specified IP addresses:

INTERNET_GROUP renderfarm 192.168.221.1 192.168.223.1 
RESERVE 2 arnold group renderfarm 

This reserves five licenses for named users:

GROUP myusergroup steve stephen steven stevie stevo
RESERVE 5 arnold group myusergroup 

You can edit the ISV options in the RLM Web Admin page. You need to restart the ISV server after you change the options. Here’s a short video walkthrough (no audio).