Although they are intimately related, Requests for Quotation, Purchase Tenders and Purchase Orders are not the same.
A Request for Quotation (RfQ) is used when you plan to purchase some products and you would like to receive a quote for those products. In Odoo, the Request for Quotation is used to send your list of desired products to your supplier. Once your supplier has answered your request, you can choose to go ahead with the offer and purchase or to turn down the offer.
A Purchase Tender (PT), also known as Call for Bids, is used to drive competition between several suppliers in order to get the best offer for a list of products. In comparison to the RfQ, a Purchase Tender is sent to multiple suppliers, stating each are competing with one another, and that the best offer will win. The main interest is that it usually leads to better offers.
The Purchase Order (PO) is the actual order that you place to the supplier that you chose, either through a RfQ, a Purchase Tender, or simply when you already know which supplier to order from.
When to use?
A RfQ is interesting when you have never purchased the products with that supplier before and therefore don't know their price. It is also useful if you want to challenge your suppliers once you have a well-established relationship with them. You can also use it to assess the cost of a project and see if it makes it feasible.
A Purchase Tender is used for public offers that require an open offering from several suppliers. It is also useful when you need to make a one-off order for a product and you would like to get the best offer, no matter which supplier it is. It may be used when your supplier has not been up to your standards and you would like to either push them to deliver a better service, or find a replacement in their competitors.
When not to use?
RfQs become unnecessary once you have established your favorite supplier for each item, and will only increase the delay in the delivery of your items. In that case, the process will be simpler by starting straight from a Purchase Order.
Purchase Tenders are a long and tedious process that will likely take more than several weeks in the best cases. If you need a quick delivery, this is not the way to go. Also, if you have a well-established relationship with one supplier, think twice before you initiate a PT with them as it might tear the relationship and finally lead to less interesting deals.
Example
My company builds wooden furniture. For the new series of table we are designing, we need some screws, metal frames and rubber protections.
I create a Request for Quotation in Odoo with those products to my usual supplier, and send it by email. He answers back with an offer. However, I am not convinced by the offer, and I want to see if anyone can give a better one.
I decide to push competition a bit and set up a Purchase Tender, that Odoo will send to a list of suppliers I specified. Out of the 8 offers I receive, one gets my attention and I decide to go ahead with that one.
I confirm the order to the supplier by creating a Purchase Order from the PT, and Odoo automatically asks delivery of the items to the supplier.
Configuration
If you want to know how to create a Purchase Order, read the documentation on From purchase order to invoice and receptions
If you want to know how to create a RfQ, read the documentation on How to create a Request for Quotation?
If you want to know how to create a Purchase Tender, read the documentation on How to manage Purchase Tenders