Found:

Complement domain

SYNTAX: Notion in checking theory. The complement domain is the subset of a domain reflexively dominated by a complement. EXAMPLE: In (i), YP is the complement domain of X (and H).

(i)    XP1
       /\
      /  \
     UP  XP2
        /\
       /  \
     ZP1    X'
    /\      /\
   /  \    /  \
  WP  ZP2  X1  YP
          /\
         /  \
        H   X2