Cyberware
Foundational cyberware
Cyberware that options are installed into: Neural Link, cybereye, cyberarm. Without the foundation, options of that kind can't be installed.
A foundation has its own slots: a cybereye holds three options, a Neural Link five. The site won't stop you from installing an option without a foundation, but it will flag the violation — the GM decides.
Related: Humanity Loss, Chipware Socket