A spool cap is the piece that attaches to the spool pin, that holds the thread in place and helps the thread properly unwind and not jump around while you're sewing.
Singer Sewing Machine Models:
1020, 1021, 1022, 1030 Creative Touch, 1036, 1040G, 1411N, 1411U, 1425N, 1425U, 2001, 2001A, 2010 Futura, 2010 Touchtronic, 248 Fashion Mate, 5502, 5504, 5505, 5508, 5522, 5524, 5525, 5528, 600, 603, 604, 611, 625E1, 625E6, 625E7, 626E1, 626E6, 626E7, 628, 635, 635E3, 635E6, 635E7, 640 Touch & Sew, 640E, 646, 648, 650, 737, 750, 755, 756, 758, 770, 775, 778, 800, 834 Stylist, 838, 844, 900 Futura, 900-3, 920, 920-3, 935, Athena 2000 (2000A)
Other Part Numbers: 163149-001, 163149-002, 163149-003, 163149-077, 163149-677, 176655
1020, 1021, 1022, 1030 Creative Touch, 1036, 1040G, 1411N, 1411U, 1425N, 1425U, 2001, 2001A, 2010 Futura, 2010 Touchtronic, 248 Fashion Mate, 5502, 5504, 5505, 5508, 5522, 5524, 5525, 5528, 600, 603, 604, 611, 625E1, 625E6, 625E7, 626E1, 626E6, 626E7, 628, 635, 635E3, 635E6, 635E7, 640 Touch & Sew, 640E, 646, 648, 650, 737, 750, 755, 756, 758, 770, 775, 778, 800, 834 Stylist, 838, 844, 900 Futura, 900-3, 920, 920-3, 935, Athena 2000 (2000A)
Other Part Numbers: 163149-001, 163149-002, 163149-003, 163149-077, 163149-677, 176655