A "510" drip tip would be one which complies to the "standard" of a 510 atomizer thus allowing you to drip into it instead of using the "cartridge" that is included. This is a very real thing. Just try to insert a 801-sized drip tip into a 510 atomizer - it will be a no-go as the 801 barrel diameter is larger than that of a 510.
edit: To add - 901 and 510 drip tips are usually interchangeable due to the similar/(maybe the same) barrel diameters of those atomizers.
edit: To add - 901 and 510 drip tips are usually interchangeable due to the similar/(maybe the same) barrel diameters of those atomizers.