No, aftertouch is where you press a MIDI keyboard a bit more after the initial finger down on the key, and it sends some more data depending on how hard you press. Only some keyboards have this. It can be quite useful for adding expression during the note.
Release velocity is sent when you let go of the key, ie note off. It can be used in EXS24 to trigger a second sample.
e.g. a good use is to emulate a harpsichord, when you let go of the key after a playing a note, you hear the quill noise.
Another use would be to sample a brass instrument and loop a section, then trigger the very end of the note.
To do this, in the EXS editor select group then make sure View>Group Voices is enabled. Her you can set the trigger for a sample to be Key release rather than the normal Key Down. You can now use vel range to assign that group of samples to play at whatever the release velocity is, ie how quickly you let go of the key. (I think - someone please correct me if I'm wrong about that, it's a long time since I did this in EXS)