Post length (also known as post height) is the length of the header pin above the plastic insulator that mates with the socket in pluggable applications (only applicable to pin headers).
Tail length is the length of pin below the insulator that is used to solder to the PCB. Applicable only to thru hole pin headers and sockets.
GCTs range of board to board headers and sockets offer a variety of customisable post and tail lengths. Please check product drawings for more information.