According to the tooltips on the Lockheed Martin site, the flight rules are:
- MVFR - Military VFR; restricted to military flights
- MIFR - Military IFR; restricted to military flights
- YFR - Initially operated under IFR, followed by one or more changes of flight rules
- ZFR - Initially operated under VFR, followed by one or more changes of flight rules
There's very little information on MVFR and MIFR flight plans that I could find, but the FAA's FSS procedures do include specific actions that are required when receiving a military flight plan (section 6-4-7).
YFR and ZFR are ICAO rules; the FAA calls combined VFR/IFR flight plans composite flight plans (see section 5-1-7 in the AIM) and on a US domestic composite flight plan both VFR and IFR should be checked. Interestingly, the Lockheed Martin site doesn't allow that.