Hi
@k8zgw! Like as has already been mentioned, I use the cross-band repeat function on a Yaesu FTM-500DR mobile/base transceiver with my HTs to access repeaters from inside the house that I could not reach otherwise. When I am in the hamshack, I do not use cross-band repeat since the Yaesu FTM-500DR can access the local repeaters using medium power (25 Watts). But when I am away from the hamshack rolling around the house in my wheelchair with an HT, the 5 Watts of transmit power is not enough to reliably access any repeaters. That is when I use cross-band repeat via the Yaesu FTM-500DR rig.
As to whether I do cross-band repeat from 146 MHz to 440 MHz or from 440 MHz to 146MHz, that depends on which repeater I am set up to operate on. If the repeater is in the 2 Meter band, you access the cross-band repeat rig on the 70 cm band. If the repeater is in the 70 cm band, you access the cross-band repeat rig on the 2 Meter band.
One final note on cross-band repeat operations. Be sure to operate according to your local frequency coordinator's band plan. I have noticed that different areas in the States use different segments of the amateur bands for low-power, inside antenna, cross-band repeat functions. I keep my HT on the lowest power possible (0.5 Watts) which is sufficient for operations within the house.