The basic structure of a spotting scope consist out of a spotting scope body and spotting scope eyepiece. User of a spotting scope can be either a first time beginner or a professional advanced user. For that matter manufactures produce and offer to sell spotting scope body and an eyepiece as separate parts. While buying spotting scope body is not such common thing in practice, buying an eyepiece on the other hand is. People normally buy the second or the third eyepiece after using the primary one for some time.
Different types of eyepieces offer diverse characteristics. Some feature variable magnification (are more useful), other offer fix magnification (are brighter). You can also find eyepieces designed for digiscoping or eyepieces that have longer eye-relief for spectacles wearers. Another thing important to mention is there are eyepieces that feature cross in the center. Such tactical eyepiece combined with a spotting scope, enable user to use the spotting scope as a target spotting scope.
The last important thing to point out is that magnification of a whole spotting scope set depends on the body of a spotting scope. If you put an eyepiece with a certain magnification on a two different spotting scopes bodies, you will get out two different magnifications. This means that smaller spotting scope body gives smaller magnification and bigger spotting scope body enables bigger magnification with the same eyepiece.