If you can't find the definition for a given dimension in the glossary, and you are still unsure of what it is meant to control, do feel free to ask me, but also consider entering some incredibly large or small value as a test. By observing how the bike changes when you enter a really big or really small dimension, you may come to understand the meaning of a dimension. Also, consider finding that dimension in the Dimensions dialog box and displaying that while adjusting your design.