It depends a what the second number is.
Flightumbers that start 88XX are extra flights are flown for example when there's bad weather in SFO and they have to cancel a lot of UA Shuttle flights. Then they will fly a 744 as 88XX to get the pax there. There could be other reasons for extra flights. The777Man
Boeing 777s flown: UA, TG, KE, BA, CX, NH, JD, JL, CZ, SQ, EK, NG, CO, AF, SV, KU, DL, AA, MH, OZ, CA, MS, SU, LY, RG, PE, AZ, KL, VN, PK, EY, NZ, AM, BR, AC, DT, UU, OS, AI, 9W, KQ, QR, VA, JJ, ET, TK, PR, BG, T5, CI, MU and LX.. Further to fly.. LH 777