O-Ring seal OR-21X2.50-NBR80

How to read the reference of an O-ring?
Example: 
OR-40X3-NBR90 
Where 
 OR = product type (O-ring or O-ring seal) 
40X3 = inside diameter X thickness 
NBR = matter 
90 = shore hardness  
Shore hardness is a material hardness scale, usually between 70 and 90 for O-rings. The higher the number, the harder the material.
What does the number mean at the end of the reference?
This is the indication of shore hardness. Shore hardness is a material hardness scale, usually between 70 and 90 for O-rings. The higher the number, the harder the material.

Difference between NBR, FPM, EPDM, MVQ?
These are the different types of rubber. Each material will be more or less suitable for a use. Refer to the following information: FOR OILS NBR = Nitrile = Butadiene rubber - Acrylonitrile
        from -30°C to +120°C FPM = Viton = Fluorinated Rubber
        from -20°C to +200°C MVQ = Silicone rubber
        from -50°C to +210°C FOR HYDROCARBONS NBR = Nitrile = Butadiene rubber - Acrylonitrile
        from -30°C to +120°C FPM = Viton = Fluorinated Rubber
        from -20°C to +200°C FOR BRAKE FLUID EPDM = Ethylene Rubber - Propylene - Diene
        from -40°C to +150°C FOR WATER EPDM = Ethylene Rubber - Propylene - Diene
        from -40°C to +150°C  FOR MORE DETAILS ON THE MATERIALS, THEIR STRENGTHS AND LIMITS OF USE, REFER TO THE COMPLETE DESCRIPTION ON THE SEALED SEARCH PAGE
How to choose the size of my O-ring?
An O-ring should not, in general, be too stretched. It is therefore preferable to take a dimension as close as possible to the groove diameter, if applicable.