A "visit" is defined as being an on-sky science observation that includes a sky region in its definition. This is a very restricted definition of grouping and a new concept is needed.
During observing a "visit" is defined by a single call to the takeImages command. Independently each observing script is allocated a group name by the script queue. This group name is stored in the headers and will be (effectively) fixed for all exposures taken by that single execution of the script (there is an API to allow a script to modify the group name to make subgroups).
Allowing a group dimension would let us collect DDF observations where we take multiple visits in a single script and would allow calibration observations to be grouped (currently they can not be grouped and people must try to remember sequence number ranges).
It may be that "group_system" is needed to allow people to modify grouping schemes.