Static versus automatic labels
There are two types of labels:
p4 tag allows you to add and delete file revisions from a label.
p4 labelsync allows you to use the named label to apply the current contents of the client to the label.
Automatic labels perform much better than static labels when synced because they are aliases for changelists.
Automatic labels using a changelist revision do not require storing each file revision, which greatly reduces the amount of data that must be stored and scanned when referencing the label.
Note that when executing the command