The VMM and the Admin guide describe how to assign/unassign affinity labels to hosts but no documentation on how to create them. VMM should have short section describing how to create Affinity labels (VM > Affinity Labels > New).
Accepting into GA program and assigning to Billy for review.
Published at https://access.redhat.com/documentation/en-us/red_hat_virtualization/4.2/html-single/virtual_machine_management_guide/#Adding_an_Affinity_Label