holy place = sanctuary or holy of holies. In this chapter "holy" is used for "holy of holies" without this adjunct. See verses: Lev_16:3, Lev_16:16, Lev_16:17, Lev_3:20, Lev_3:23, Lev_3:27. Compare note on Exo_3:8.
mercy seat. Figure of speech Antemereia (of Noun), App-6, noun, "mercy", used as adjective. See note on Exo_25:17.
I will appear = I am wont to appear. See Exo_25:22.