Direct Answer
An addendum is a formal modification to the contract documents issued by the design team before contract award, while an RFI is a question (and response) issued after contract award. If a design change is issued as an addendum after you've signed your subcontract, it is a scope change and typically warrants a change order. RFIs clarify existing design; addenda modify the design documents.
Full Explanation
Addenda are most commonly issued during the bidding period to correct or add to the bid documents. A fabricator who signs a contract after an addendum is bound by the addendum as part of the contract. Addenda issued after contract award are typically called 'Supplemental Instructions,' 'Bulletins,' or 'Design Bulletins' and function like addenda but trigger change order rights since the contract price was established before the change. Always review every addendum or bulletin against your original scope to identify cost impacts.
What This Means for Your Shop
Read every addendum or bulletin carefully — even ones that appear to be architectural. A change to a floor slab profile, for example, may change your shear stud layout or your deck opening details, impacting your scope indirectly.
Common Mistakes
Assuming addenda issued after contract signing are not cost-impacting because they're 'just design updates.' Any scope change after contract execution is a potential change order, regardless of how it's labeled.
Recommended Process
When you receive any addendum or bulletin, compare it against your original drawings and scope. Identify every impact on your work. If cost-impacting, submit a written notice within the timeframe specified in your contract (often 10–21 days).