A RAMTAF is a weather report (TAF) for a station that does not have a regular forecast. In my experience, our company had RAMTAFs prepared for us by Kavouras, for several of the mountain airports that we flew into in the Rockies. This is a service that your company pays for.
In addition to what Flightracker said, the RAMTAF forecasts tend to be less restrictive than the NOAA/NWS ones. They seem to be more favorable than the others in certain circumstances, so they are used instead when the official forecast is marginal. This is legal for most airlines as it is authorized in the op. specs. The release only has to say that RAMTAF was used in those cases.