Pak at the wrist; was mainly to deal with jab "style" punches, were brief, to the point, didn't always follow Wing Chuns concepts, but saved my face alot lol.
With jab type punches, you will find that you can't always get to the elbow in time, tried... Failed many times, hangs head in shame. Unfortunately that is reality at times, we train to put our opponent into our box, basically our playground as a Chunner, but those darn brutes don't always stay in the sandbox or even play nice for that much.
At the forearm; this idea was more of a Kiu Sau technique. We would start at the mid-forearm area, then either slide to the wrist or elbow depending on the energy, & motion of the attack. More of a way to try & gain control of the bridge, rather than a technique itself.
At the elbow; the elbow, we'd rarely pak the elbow(slightly above) it was more a follow up pak, after engaging with a biu sau or jong sau. Or in closer ranges, a way to control the limb & then follow up with ground work, i.e. Take down, kam na etc.