Provenance
Where it comes from.
'Placement' is the younger of the two words, coined in 1835 from the verb 'place' plus '-ment,' the act of putting something, or someone, in its proper position. 'Place' as a verb dates to the mid-15th century: to determine where a thing belongs and set it there, and by the 1590s to find a person a situation or a home.
'Key' is far older, from Old English caeg, a metal piece that works a lock, and of famously uncertain origin. Its figurative sense is nearly as old: even in Old English a key was 'that which opens the way or explains,' and by the 1520s 'that which holds the other parts together.' Set beside 'placement,' it promises the piece that unlocks the right match.