So what are the "Assignments" in CADSTAR for?
Basically assignments is the term given to they settings for all the route, line, pads, text, copper, via codes amongst others. You assign values to them.
They are all kept within the assignments in CADSTAR, it is worth noting that CADSTAR keeps its assignments in each individual design file, within each schematic and documentation symbol and PCB library also the default files used for new design files.
So essentially they are configured in many places, so which set is the master set?
Nether really, as you are able to change them specifically for each individual design however it is always advised that your library assignments should be treated as the master set and they should overrule any others.
When creating your assignments you should always try and name them as descriptive as you can for their purpose, for instance - for the outlines on symbols call the assignment "Symbol Outline", for the text code used for Pin Labels then call the assignment "Pin Label Text".
Create an assignment for every type of item used in CADSTAR so that changing a single assignment does not mess up other items.
I.E. if you used (the rather nondescript named) "Line 10" for both symbol outlines, figures and document symbol outlines then a change from 10 to 20 just to make your border look thicker will make a mess of all your symbols. In my opinion it is much better to have a greater control of your items by giving them individual assignments.
I would also suggest that you document your assignment names, define exactly what they are to be used for and when.
That way you are able to copy and paste from an electronic document the names of the assignments so that you do not create 2 similar named assignments but with an extra space or typo in, as it it very hard to remove an assignment once used in your library.
In Settings\Assignments you have separate tabs for each type of assignment, there are too many to describe in one post so I wont go into them that deeply, suffice to say that you should learn what each one is for, create and stick to a theme for your assignments.
As a note - I have said that you are able to change assignments in each design file, this means that for instance you can select a pad, choose it's properties, go to the assignment for that pad and change it's dimensions. Be extremely careful doing this because if you download the assignments or reload the component you are likely to revert the assignment to how it originally was. There are better ways to change pads.
Finally once you have figured out what assignments you want to use and why you want to use them it is also sensible to document them. They can be copy and pasted into other documents to aid this.