Just saw this.Ace Seeds calls their Malawi Standard "P4 landrace from Malawi, Central Africa. Old killer Malawi x 3rd generation New Malawi Killer."
What does P4 mean?
I remember on their site they sometimes referred of them as "pure" varieties as well. I think they differentiate pure and stabilized landrace as the P variety being the parents and then they stabilized with fillial like Weaslecracker said.
ACE is a loose association of breeders made up of enthusiasts growers and landrace afficinados so it helps them to have a sort of "pedigree" established. So for example someone that's been growing ACE seeds for a long time may have seen them switch from using Old Malawi Killer to New Malawi Killer, and so it might help people point out different growing needs or effects from the strain.
Another example is their golden tiger. It use to have more Thai influence, so now golden tiger F1 will be a little different than what they have as golden tiger now