In a collision between an electron and a hydrogen atom, it is useful to select both objects as the system because which of these:

1
the total momentum of the system does not change during the collision.

during the time interval just before to just after the collision, external forces are negligible.

the kinetic energy of a two-object system is nearly zero.

the sum of the final kinetic energies must equal the sum of the initial kinetic energies for a two-object system.

the forces the objects exert on each other are internal to the system and don't change the total momentum of the system