 the
incoming pod labels, those key-value labels are ANDed with labelSelector
to select the group of existing pods over which spreading will be calculated
for the incoming pod. The same key is forbidden to exist in both MatchLabelKeys and LabelSelector.
MatchLabelKeys cannot be set when LabelSelector isn't set.
Keys that don't exist in the incoming pod labels will
be ignored. A null or empty list means only match against labelSelector.

This is a beta field and requires the MatchLabelKeysInPodTopologySpread feature gate to be enabled (enabled by default).²