Arrival Group Letter significance?

[13cheermom]

So we are new to Princess and just chose an arrival time.  However what is the difference or meaning of the different group letters vs. the boarding times?  We chose a 11:00 - 12:00 time however it says Group I, so what actually does that mean?  Looking for all you experienced Princess cruisers.  Thanks!

[cr8tiv1]

Groups are split:  Group A/Group 1/11 AM) and Group A/Group 2/11;30 AM.  Or so I think.

 

Most, if not all, embarking groups have not been scrutinized for time since ships are sailing at about 50% capacity.