From my experience, when we first got the MXT400's, they only operated on simplex until you change the menu to allow repeater channels. This just adds additional duplicate channels, so that you have both simplex and repeater channels available. After doing this, as you rotate (or scan) through the channels, it displays the lower power channels 1 - 7, then the simplex channels 15 - 22, and then the repeater channels 15 - 22. Thus, if you transmit on a simplex channel, both rx and tx are on the same frequency, but if you transmit on a repeater channel, the required 5 MHz transmit offset is preset.
It is different than what I have used for ham radio where a given frequency can be either simplex or a plus or a minus offset, or even a non standard offset. With GMRS since there is only the required 5 MHz plus offset and I suspect they did this to make it simpler for non technical users.
If I am scanning, I will hear repeater outputs on either the simplex or repeater channels (15 - 22) assuming I have the correct tone (PL or DPL) set or no tone. If I then wish to stop scanning and use a repeater, I have to remember to use the repeater channels which display an rP on the lower left of the screen.
The one shortcoming is that if you set a tone, it works for both TX and RX. I have never come across this in ham radio, but it seems that some GMRS repeater owners use different tones for input and output. If you really want to use a given repeater, the work around is to open the squelch for receive and ignore the repeater output tone, but, of course transmitting the access tone to the repeater. This means that if you are scanning, you will not hear the repeater if you have the tone set. If you do not have a tone set, you would have to select the tone before transmitting in order to access the repeater. This is a bit awkward but could be done.