Cisco 12000 - Series Chassis Modular Expansion Base Hardware Installation Manual page 128

Glossary
Shared port adapter. A SPA is a modular, platform-independent port adapter that inserts into a subslot
SPA
of a compatible SIP carrier card to provide network connectivity and increased interface port density.
The SPA provides the interface between the network and the SIP.
Secondary slot on a SIP where a SPA is installed. The primary slot is the chassis slot on the router.
subslot
Cisco 12000 Series Router SIP and SPA Hardware Installation Guide
GL-2
Release 12.0(32)SY1, OL-8831-01, Rev. G6, July 19, 2007