There is a fundamental difference between RFQ's and RFP's. It may not seem important to arts administrators, but that one simple letter is going to make a big difference in the quantity and quality of submissions your call receives.
An RFQ is a request for qualifications, meaning that your organization is collecting applications detailing an artist's experience in working on projects similar to yours. An RFQ does NOT include preliminary sketches or plans for the requested artwork. Asking artists to include their initial ideas or approach to public art and/or your project specifically in their letter of interest is generally well-accepted and a good practice to help narrow the field of eligible applicants.
An RFP is a request for proposals and requires the artist to submit some form of a proposal, most often this includes preliminary ideas, sketches, budgets, and maintenance instructions. Since there is no guarantee of compensation for the substantial amount of work required for an RFP, many artists view them as risky and unprofitable. For this reason, RFP's traditionally receive far fewer responses than RFQ's. However, the quality of submissions for an RFP is high since only artist who are interested in your project enough to invest the time to complete the RFP will apply.
It is imperative that you mark your call appropriately. PublicArtist.org will not tolerate mislabeling calls, intentional or otherwise.