Found:

Extension requirement

SYNTAX: (minimalist theory) the requirement that substitution operations in overt syntax always extend their target. This requirement is met iff the designated position E for GT in overt syntax is external to the phrase-marker K, i.e. extends K, unless E is an adjunct position. In effect, GT can only insert material into the highest specifier position of K*.

LIT.	Chomsky, N. (1993)