Kenneth S. Wuest, 'Ephesians and Colossians in the Greek New Testament', Wm B. Eerdmans Publishing Co. Grand Rapids, Mich, 1963, p30:

The main import is appointment for a certain object or goal;

third, the word is used of the act of choosing some person or thing for a definite object or calling. The middle voice in Greek represents the subject of the verb acting in his own interest or for himself...