Difference between RFI, RFP, RFT and RFQ in tender process

What is the definition and difference between:

RFI - Request for information
RFP - Request for proposal
RFT - Request for tender
RFQ - Request for quotation

The differences between each of the requests are quite essential. Purchasers use them in tendering process to select the best vendor. In some cases, to choose a contractor, only one of them is being used. It may also depends on the situation on the market.

RFI - Request for information is used for gaining information, preparation for an RFP or RFQ, forming a strategy or building a database with a purpose to identify potential vendors. It is often used when there are many potential suppliers. Procurement may use RFI to include a list of products for pricing requests. Sometimes it is used mainly for the registration of interest.

RFP - Request for proposal is a formal method of receiving detailed and comparable proposals from potential suppliers. It provides to procurement required information, that is needed to make an informed purchasing decision. RFP usually follows an RFI. Purchasers using RFP are often seeking solution based offers to meet their needs. In comparation to RFT, it is characteristic with greater flexibility.

RFT - Request for tender is a formal invitation to potential suppliers to bid to supply clearly defined products. It is focused on price and also on quallity.

RFQ - Request for quotation is used when inviting potential suppliers to submit a bid on specific products or projects. The request is targeted primarily on pricing.