Socket Countersunk Screwscome as 'sets' and 'bolts'. The, ‘SET’, term is used to describe a fastener that is fully threaded in lengths up to and around 1-2 inches, from the underside of the head (widest point of the head). Over 1-2 inches in length the fastener will have a, 'shoulder,' or unthreaded area under the head (plain shank). This space of unthreaded area confirms that the fastener is now a bolt (cap head, bolt in this case) and is shouldered. Some people might simply call them Allen or, 'Allan bolts,' in association with the Allen, Hexagonal Key which is used to tighten and loosen the fastener. So, ‘Socket,’ refers to the hexagon shaped indentation found on the top of the fasteners’ head.
The length of a countersink fixing/ fastener is always from tip to the top.