Pretty much it's about sharing expenses. But if I go by myself from A to B, neither expense sharing or common purpose applies. But then B to A is taking my mom/

Pretty much it's about sharing expenses. But if I go by myself from A to B, neither expense sharing or common purpose applies. But then B to A is taking my mom/friend as well as myself back home. So that voyage is common purpose and if there are two people the cost of that direction can be split 50/50 (or I can pay a bigger share, I just can't pay less).