There's no clear cut answer to this question. If you look at FCRA, you'll see there is nothing in the act itself which defines what a firm offer of credit consists of. So, there's nothing really to point to except the absense of statutory requirements.
You may be aware there is ongoing litigation regarding how specific a firm offer of credit need be. Certain courts have really muddied the waters by holding that a firm offer must be reasonably specific as to the terms being offered, and even must offer a consumer 'value.' Most of these cases have focused on credit terms as opposed to line, although one case [Cole vs. US Capital] did seem to consider that an offer which might have been as low as $300 for an auto loan was not a firm offer. In another case [Simpson vs. Juniper], the bank was denied motion to dismiss and the court opined that minimum credit line being offered is important in determining whether an offer has 'value.' In the words of that court "Upon receiving Juniper Bank's offer, Plaintiff had no clue if the offer was for a credit line of $5, $100, or some other amount of money." Most critics agree these courts are going well beyond the FCRA in establishing requirements for a firm offer. The type of credit being offered [i.e. auto vs. credit card] seems to matter on the topic of line availability. If you don't know the specific line being offered, it may make sense to begin disclosing at least a minimum line to be in a safest position.
It would be helpful to know what "someone" is asking this question; customer? coworker? regulator? I'd obviously deal with what appears to be a potential plaintiff on a fishing trip very carefully. Many (most?) creditors use a combination of bureau/application data to determine line assignment. Income is usually a key contributor, and it is not possible to get income during a prescreening. If your line assignment is not based purely on bureau data, I think your easy answer is just that... you need application information to establish final credit line. Goes without saying you should be careful in any response you put out there based on ongoing FCRA litigation.