The agreed list of which metal and via layers are fixed in the base and which are made for each customer, with the rules and limits the custom layers must keep, agreed with the foundry.
Specification Work product in the Structured ASIC lifecycle
The layer split is the core of a structured ASIC: it decides how many masks each customer pays for, how fast wafers return and how much freedom the design has. It must be agreed with the foundry because the base wafers are processed and banked at the point it defines.
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