X18 Supply Contract

Under the supply contract, there are selected secondary clauses that are not included/able to be used.

Why is X18 not able to be used?

I presume it wasn’t considered necessary by the drafters. It could always be introduced as a Z clause if you wanted to cap liabilities.

For the SC form, if you are referring to NEC3 then X18 is not an Option, however it is an Option within the NEC4 version.