An RFP (Request For Proposal) is a process by which a buyer gathers information from potential suppliers (respondents).
This process can take one of two forms: "open" or "closed".
In an open RFP, any potential supplier is allowed to view the RFP documents and submit a response (sometime called a bid).
Open RFPs are more common in government environments, where rules are in place to ensure that purchases decisions are unbiased.
A variant of an RFP is an RFI (Request For Information). An RFI is typically issued earlier in the purchasing process, and is used to learn more about the purchase category by collecting information about the range of products or services on offer. RFI's usually solicit much shorter responses, and are often "open", even in the corporate sector.
A closed RFP is issued to specific, invited respondents only. Submissions from other respondents will usually be ignored. Closed RFPs are usually issued by for-profit organizations that already have considerable knowledge of the good/service being purchased, and who have identified a short list of potential suppliers.
How does this work in PostRFP?
When a Project is created in PostRFP it can be flagged as "Public" or "Private" (the default). A public project corresponds to the open RFP process described above - any supplier can view the RFP and submit a bid.
The terms public/private are used because Projects in PostRFP pass through a number of status transitions: Draft -> Open -> Closed. To avoid confusion we use the terms public/private rather than open/closed.