Requirements and functional specification
The requirements specification describes what the client needs; the functional specification describes how the supplier will implement it.
The requirements specification is produced by the client and formulates requirements from a business perspective — ideally solution-neutral. It answers which problem should be solved, not with which technology.
The functional specification is the supplier's response. It describes the planned implementation concretely enough that both sides share the same understanding of the outcome, and forms the basis for effort estimation and acceptance.
In ERP projects this effort need not balloon. What matters is less the size of the document than clarity in the places where disputes later arise: interfaces, data migration, permissions and acceptance criteria.
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