Hello @ChristofAbt,
PTA0 and PTA3 corresponds to SWD standard signals:

And are used in the SDW connector:

These are designed to support JTAG and SWD, also, as mentioned in the chapter 3.2 called "Series and debug adapter (OpenSDAv2.1)" in the Freedom FRDM-K66F Development Platform User’s Guide. These signals are also brought out to a standard 10-pin (0.05”) Cortex Debug connector (J9). I highly recommend see this chapter in order to obtain more information.
BR
Habib