About “Region” & “Group”

What does "distinguish" mean? To count them, to label them,
to operate on then in some fashion? Groups are "static"
in that atoms are assigned to them and stay in that group
no matter where they move. Regions are more dynamic. When
you test, the region condition is checked for at the time
of the test.

So you can simply put your initial atoms in one group. Have fix
deposit add atoms to another group. Then anytime you can,
for example, count the atoms in one group and that are in
a particular region. You can do this with a region that
invokes the count(group,region) function - see the variable command
for details.