A preliminary version of the client’s RFP, usually issued to solicit industry feedback (for example, on the risk that industry is being asked to assume, the package of Work being requested, the standards being required, the pricing methodology) and to ask for alternative solutions that might otherwise be ruled non-compliant.
Often incomplete: most typical (and entirely deliberate) omission is the response instructions and evaluation criteria, since early release of those would give bidders a head start on the proposal.
Sometimes issued in a genuine effort to improve a procurement; sometimes appears to be issued as a pro forma response to an internal requirement to consult with industry.