There are a number of ways to consider the Volume of a Vial.
One way is if you fill the vial all the way up until there is no headspace when the cap is secured. You could call this a 2mL vial. In practice though, a sample cannot contain this much volume because of displacement caused by the syringe needle during injection.
For this reason, a 1.8mL of 1.5 “useable volume” designation is sometimes stated instead. This is known as a “nominal” volume or simply a name.
Why is this important? If you fill the vial to the brim, it will contain approximately 2ml. If you put a needle in it to remove some of the liquid for injection, the needle will displace the liquid and it will spill over the top. If a cap is on the vial, liquid will squirt out of the hole the needle makes. So the volume for autosampler filling is called a nominal volume.