If you have not heard of the “New Studies in Dogmatics” series, edited by Michael Allen and Scott R. Swain and published by Zondervan, I would suggest you look it up (and perhaps buy a volume!). In the second volume of the series, The Triune God, Fred Sanders aims “to secure our knowledge of the triune God by rightly ordering the theological language with which we praise triune God. Its central contention is that the manner of the Trinity’s revelation dictates the shape of the doctrine; it draws its dogmatic conclusions about how the doctrine should be handled on the basis of the way the Trinity was revealed.” Here’s just a small, golden nugget from his section on the distinction of persons in which he talks about divine processions and divine missions:
“Neither of the sendings is a new relation of origin in addition to the processions. Each is instead the incorporation of a further ending point to an eternally existing procession. The whole idea of a mission, as Aquinas clarifies, involves an origin point and a termination point. The origin point of a divine mission is not different from the origin point (if we may speak this way) of an eternal divine procession. That is, the eternal Son is eternally from the Father as his origin point, and the incarnate Son is identically from that paternal origin point…. But the Son does have two distinct termination points, one in the divine nature and one in the human nature. The Son’s termination point in the divine nature is his eternal deity; his termination point in the human nature is his assumption of humanity in the finite and temporal world of the events of salvation history. In Aquinas’s own words, ‘Mission includes an eternal procession, but also adds something else, namely, an effect in time; for the relationship of the divine person to a principle is eternal. We speak therefore, of a twofold procession––the one during eternity, the other during time––in view of the doubling not of relation to principle, but of the terminations––one in eternity, the other in time.’
Aquinas speaks here of a ‘twofold procession,’ which is terminologically unwieldy because it seems to call the mission a procession. But distinguishing mission from procession is the whole point. The reason Aquinas calls this movement from the Father through the eternal generation to the incarnation a ‘twofold procession’ is to make the point that the Son is from the Father by way of procession that terminates in deity, and (as incarnate) is also from the Father when that procession becomes twofold by terminating in humanity. For clarity’s sake, it is much more helpful to reserve the word procession for the immanent action, and to use the word mission to designate the external extension. When attending to the identity of the origin point, the single source of the fromness of the Son, we can recognize that the line form the eternal Father to the incarnate Son is a straight one…. The line from the Father to Christ incarnate passes through the eternal generation of the Son.”
Sanders, Fred. The Triune God. New Studies in Dogmatics. Allen, Michael and Scott R. Swain gen. eds. Grand Rapids: Zondervan, 2016.
 Fred Sanders, The Triune God New Studies in Dogmatics (Grand Rapids: Zondervan, 2016), 19.
 Ibid., 125. Quote of Thomas Aquinas, ST, 1.43.a.2.