Theses 2 number are the size of the bottles
48CU = 0.8 Liters of air
68CU = 1.1 Liters of air
The bigger the CU the bigger the bottle. Bigger bottles = more air storage
Steel bottles allows you to fill to 3000 psi
Carbon fiber allows you to fill to 4500-5000psi depending on the bottle
The higher the psi = more air storage
Steel bottles are cheap so the avarage person buys a 68cu - 3000psi bottle. This allows you a good amount of storage
All bottles have a test date. A test date is when the bottle was pressure tested. This makes sure it's safe to use
You don't want this to happen
If your buying second hand be carefull with bottles. It's best to buy new just incase