Increasingly clients/customers either know what they want, or firmly believe that they know what they want; sometimes they do not disclose their requirement until they have made a firm internal decision. Whichever it is doesn’t really matter – for the supplier the key is simply to politely and helpfully answer the question whether the question is informal, by email or within a detailed Request for Proposal (RfP), Invitation To Tender (ITT), or similar.
If the question isn’t right then by all means discuss how it can change with the customer, although caution should be adopted to not seem too pushy or superior and sow-the-seed that you could be a difficult supplier to work with.
The skill is in giving the customer what they ask for and costing and pricing accordingly. This way, if things change so can the price. Also, the risk of losing on price (by being too clever or insisting on what the client may see as needless extras) is lowered.