I’m not sure if there is an official definition of a park flier, but in general I’d say any plane that can be “safely” flown in a small space qualifies. This generally means smaller, light weight models with electric power. TX/RX has nothing to do with it.

The 72Mhz aircraft band is public use. Anyone can use it anywhere at any time for it’s intended purpose, as long as they don’t cause interference with other bands/uses as outlined in the FCC regs. The “freq pin” is a self-enforced thing the AMA does. The AMA and its members do this for obvious reasons, but there is nothing legally preventing anyone from transmitting without a pin, even if they are next to an AMA field. This can be troublesome if non-AMA members fly park fliers close to AMA fields as there is little the AMA field can do legally, since the band is public. The flip side is if you fly at your own risk, and in uncontrolled areas, the risk of being shot down may be higher if others are flying since there will be no frequency control in a general park area that all agreed to adhere too.

Minor point on age’s post, the “toys” on other bands, such as 27MHz, are not ground frequencies. It is illegal to operate a flying device on ground freqs. The 27MHz band allows all craft type use as per the FCC, but this doesn’t make them ground freqs. It makes it an air/ground/water band. A ground freq would be a band with use exclusively noted for land/water vehicles, such as the 75MHz band. Likewise, the 72Mhz band is air only. Planes on 27MHz are legal. But Operating an aircraft on 75MHz would be illegal, but I don't know of any being sold that way.

Cheers.