For each existing bone which has a Mirror symmetric bone defined (e.g. mCollarLeft – mCollarRight are mirror symmetric bones) Avastar checks if both bones of the pair have weightmaps defined and creates the maps if they do not yet exist.
In the image we see weight maps for mCollarLeft and mShoulderLeft while their symmetric counterparts are missing.
After clicking on Add missing Mirror Groups the weight maps for mCollarRight and mShoulderRight have been created.
Important: The created new weight maps are still empty. You still need to create or copy the weights.