Firstly, the Activity Schedule is not used to specify any product, which is the job of the Works Information.
Where a particular product is not specified then the designer would be free to choose whatever product they wanted, provided it complies with any stated performance requirements.
If the Client (Employer) wishes to specify a particular product 'post-contract' then this would be a change to the Works Information which should be instructed accordingly by the Project Manager.
It is possible that there won't be a cost implication but the Contractor would be given the opportunity to assess factors such as; product availability and lead times, the delivery, storage, handling and application/installation requirements, testing, etc, which could influence the Defined Cost of a product, so quoted unit cost would only be part of the quotation assessment.
Where a particular product is not specified then the designer would be free to choose whatever product they wanted, provided it complies with any stated performance requirements.
If the Client (Employer) wishes to specify a particular product 'post-contract' then this would be a change to the Works Information which should be instructed accordingly by the Project Manager.
It is possible that there won't be a cost implication but the Contractor would be given the opportunity to assess factors such as; product availability and lead times, the delivery, storage, handling and application/installation requirements, testing, etc, which could influence the Defined Cost of a product, so quoted unit cost would only be part of the quotation assessment.