The Bases category covers a wide range of accessories that have the overall aim of allowing you to interface between scope or sight, mount rings and gun base in the way you want.
The most common base is when the gun model has a base that there are no suitable mount rings for due to it being a special receiver or, the sight (normally a red dot or night vision) having an integrated picatinny base that requires a picatinny receiver.
Other common applications are the riser – which is same base to same base fit (mostly picatinny) in most cases and is adding height for reasons such as eye relief and/or shooting position.
The other main sub type is the red dot base for pistols where there are a number of specialist base fitting types.