SAP is (was) a data field- part of the OSI layer 2 LLC frame. It originated from the old DEC (Digital, Xerox, & Digital Equip. Corp) ethernet standard.
Its' function was to identify what type of higher level protocol (that would be the next level up- or 'Network' layer, in the OSI model), so the SAP field might contain a code for - IP or IPX.

Modern encapsulations, AFAIK, have changed the SAP field to a 'length' field.. conceptually, it informs higher levels of the OSI model as to the length of the Layer 2 payload, instead of the 'Type' of payload, as the original SAP field did.

Google IEEE 802.2, Ethernet v2, Ethernet 802.3.

P.S.
-justed checked my bookshelf... I remembered where I first came across this material.

Book:

CCNA Self-Study
CCNA Intro
Exam Certification Guide
by Wendell Odom
Cisco Press
ISBN 1-58720-094-5