The aspect of sanctification accomplished once for all at conversion: the believer's decisive separation to God and from the world in Christ. Paul regularly addresses New Testament Christians as "saints" (hagioi, "holy ones") — even the scandalously-behaving Corinthians (1 Corinthians 1:2). He is not being ironic. He is describing their position before God: set apart, declared holy, belonging to God. Positional sanctification is distinguished from progressive sanctification (the ongoing transformation) and final sanctification (glorification).
The three-tense structure of sanctification is critical for Christian assurance. Scripture teaches: (1) past — "you were sanctified" (1 Corinthians 6:11, aorist tense, a completed event at conversion) — this is positional sanctification; (2) present — "being transformed into the same image from one degree of glory to another" (2 Corinthians 3:18) — this is progressive sanctification; (3) future — "when he appears we shall be like him" (1 John 3:2) — this is final sanctification or glorification. Confusing these tenses destroys peace. If you think your acceptance before God depends on your progressive holiness, you will either be proud (on good days) or despairing (on bad days). Positional sanctification says: God has already declared you holy in Christ. That status is not improvable. Your progressive holiness flows from your position, not toward it. "Become what you are" is the Pauline imperative — the indicative (you are a saint) always precedes and grounds the imperative (therefore live like one).