The quality required of an item or service is stipulated in a specification. Frequently a brand name may be used. This is to express the quality required, and not a preference for a particular name brand item.
Vendors with products meeting or exceeding the quality requirements are invited to quote even though the brand name of their products may not be mentioned in the specification.
Sometimes a special brand name and model may be required due to reasons of compatibility with existing equipment. In these cases only that item will be accepted.