There are two common backsets for residential  locks,  2-3/8" and 2-3/4".

The backset is the distance from the edge of the door to the center of the 2-1/8" bore hole. Locks will be packed with a  2-3/8" or 2-3/4" latch depending on which backset you specify.

Most locks come with adjustable latches to accommodate both 2-3/8" & 2-3/4" backset. If you are not asked to select a backset, the backset is adjustable.

Dummy sets have no latch and are surface mounted so you can install a dummy set wherever you choose on the door, if the door has not already been bored.