X01 - X20, CS01 - CS10 Elaboration

Can someone elaborate on how to write rules for custom techniques? I've seen the idea in several posts, but I'm not quite sure I understand the full usefulness of these techniques...
Can x01 be assigned to represent a type of note head (as referenced here: viewtopic.php?f=19&t=1331)?
Can you define start techniques and end techniques?
Etc.
Can x01 be assigned to represent a type of note head (as referenced here: viewtopic.php?f=19&t=1331)?
Can you define start techniques and end techniques?
Etc.