A Key-Mapping Key is a key which maps to a Transmitter address – Receiver address combination. That is, The Key-mapping key between a particular transmitter address and a receiver address can only be used between that [transmitter address, receiver address] combination and not with other transmitter or receiver address combinations.
This is different from that of a default key where all the mobile stations share the same default key. The diagram below explains the Key-mapping keys concept.
STA1 speaks to the AP using KEY: abcd
STA2 speaks to the AP using KEY: efgh
A packet meant for STA1 cannot be encrypted using KEY: efgh as the STA1 does not have that key and will be unable to decrypt it.
Due to the complexity of implementing different keys for different stations – Key-mapping keys was not widely implemented in most Access Points.