Note that if you are using multi-city on Matrix to construct an itin where there are multiple legs priced on one fare component (basically, to force a particular routings with short layovers that can price on one fare), Matrix doesn't actually check for married segment inventory availability in these cases. If you try to call to have an agent book the itin and there is no married segment inventory for the fares Matrix is quoting, they will likely not be able to actually book it.