The Leadbuyer can activate the Offer-verification phase under the Tender properties page, under the Procedure Type tab, by clicking on the 'Edit' button and select 'Apply Offer-verification'.
Selecting this option will create an extra phase within your Tender (this will also be added to your schedule page), either after the Selection of the Offer phase. 

Creating the Offer-verification phase enables you to create an additional questionnaire specifically for this phase. Often it is used to request evidence to confirm the vendor's earlier response to requirements set in the Selection or Offer phase. This way you only have to verify the evidence for vendors that are selected instead of every vendor that has applied. 

The questionnaire in the Offer-verification phase can be manually added at the 'Questionnaires' page, by clicking 'Add' at the bottom right. Creating the questionnaire you will have to indicate the 'Period for supplier to provide answers'. Choose the offer-verification phase here. Automatic creation of the verification-questionnaire is also possible, by indicating you later want to verify a requirement listed in an earlier phase (Offer or Selection). 

For example you want to ask your vendors during the selection phase whether they work according to ISO standards. To save time you only want to verify the ISO-certification of the selected vendors. So you create a simple yes or no question in your Selection questionnaire, asking vendors whether or not they are working according to ISO standards. Creating this question you tick the box 'Verification Required'. Choose 'Save' and you will be asked you to create the Verification question.

In the verification question only the text needs to be changed, the question is already set as a 'Document Evaluation'. Formulate the text so that it is clear what proof is required. The verification question was created in a new, additional questionnaire with the name: Verification-suffix. Other verification questions created in this manner will be automatically added to this same questionnaire.

Note: the verification phase is mandatory when activated. It is a phase in which at least one vendor must submit a response in order to continue to the next phase of your Tender (Offer or Award). So please consider upfront whether an verification phase is applicable in you Tender.