The difference is the format of the data that the RX is capable of decyphering. If the transmitted signal is not in the right format, the RX will not operate the servos/gyro/etc correctly or at all. Obviously, if you have a PCM RX, you must have a TX capable of the same PCM format or it won't work. Likewise with an FM RX, you have to have a TX capable of the same FM format.

If by "heli radio" you mean just the TX then it's not a problem. Most decent TXs can transmit on both PCM and FM and is selectable in the models memory settings. You just have to make sure the settings are correct for the RX in the model. PCM also gives you fail safe options.
Example: Futaba's 9CHP and 9CHF TX's are identical except for the little label on the TX. Both TX models will transmit PCM or FM by setting up the model memory appropriately. So why the difference in labeling? Generally the TX's come as a package and the label indicates the RX type that came with the package. Has nothing to do with the TX (since it is capable of both).

Good Luck,
d.tipton