applying patches step-by-step

Exchange 2013 cu4

finally we're going to apply cu10. from what i gathered googling around, some say patches:

1. can be applied without any downtime

2. can be applied with CAS servers first, then MB servers later

i kinda have my doubts of the above as when you look at DAGs,  they act as one system. so how can patches be NOT applied at the same time? and without downtime?