What is the use of the adverbial participle ὑπάρχων in Philippians 2:6?
1. concessive: “although”?
2. causal: “because”?
3. something else?

• GNT ὃς ἐν μορφῇ θεοῦ ὑπάρχων οὐχ ἁρπαγμὸν ἡγήσατο τὸ εἶναι ἴσα θεῷ,
• NASB who, although He existed in the form of God, did not regard equality with God a thing to be grasped, [cf. ESV, NET, NRS, NLT, NAB]
• NIV Who, being in very nature God, did not consider equality with God something to be grasped, [cf. Tyndale, Geneva Bible, KJV, NKJV, HCSB]

It appears that all of the major English translations render ὑπάρχων either (1) concessively or (2) nebulously or generically (e.g., “being” or “existing”). I haven’t found a single translation that renders it causally. Cf. D. A. Carson, Basics for Believers: An Exposition of Philippians [Grand Rapids: Baker, 1996], 44-45):

• [T]he opening expression, both in Greek and in English, is slightly ambiguous. The phrase “being in very nature God” could be understood in one of two ways. It could be understood concessively: although he was in very nature God, he took the form of a servant. Or it could be understood causally: because he was in very nature God, he took the form of a servant. On the whole, the latter better suits the context. The eternal Son did not think of his status as God as something that gave him the opportunity to get and get and get. Instead, his very status as God meant he had nothing to prove, nothing to achieve. And precisely because he is one with God, one with this kind of God, he “made himself nothing” and gave and gave and gave.