Category modifiers allow you to slice & dice variables using the related categories & category items. For the 101 on Categories go here.
Once you've linked categories together, you can then easily slice variables along different categories.
Example: we have an Employee category, which is linked to a Team category. By default, we've worked out Salaries by Employee, and now we're interested in seeing Salaries by Team.
Click on the Category icon in the referenced variable in the formula, and then select the desired category and click Group By.
If you want to "Group by" multiple categories at one time just hold down shift and click each of the categories as you're selecting. e.g. if you want to group by Team and then Role. This displays as multiple layers of nesting.
Sometimes you'll want to select a single item from a category and use it in a formula.
Click on the Category icon in the referenced variable in the formula, and then select the desired category and then click on the individual item you want to filter by.
Example: if I want to do a calculation only based off the Engineering Team's salaries, you would select engineering under Team.
Note: if you want to filter for multiple category items, just hold down shift and click each of the category items as you're selecting.
Sometimes you'll also want to aggregate away a category, if you no longer care to see that level of granularity.
Example: If I've filtered Salary by Employee for just the Engineering Team, but I no longer care to see the Employee breakdown, I would aggregate the Employee category.
There are a few other ways that you can use / manipulate categories and category items within variables and formulas, that are not Variable modifiers in formulas.
Sometimes you might want to change how a variable aggregates it category items. Causal's default is to sum the category items, but see Category aggregation for more options.