Yes it is that simple.

Flow - is for the pressure switch that screws into the heater housing:

http://spapartsnet.com/Electric-Heaters ... 1_2_1.html

The high limit sensor mounts in the heater flange on the back somewhere yes, and the temp sensor can mount any place the spa manufacturer decided to put it...

http://spapartsnet.com/Electronic-Temp- ... 6_1_1.html

Don't switch them around. Actually, never switch them around.

It's dangerous, unsafe, and you're liable to cause damage to your tub or circuit board this way.

(and you could even burn your deck/house down with an incorrect setup).