Is it possible to nest two list categories?

[Gusta2307]

Can I create subcategories within a category list? I'm looking to do something like this, where A, B, and C would be subcategories.
Company: [ A ▼ ]
                 ├─ B
                 └─ C

[Brian Lancaster]

No, I think you would need to make a new field B that is depended on A and then make C depending on B. Best practice however would just be to have a category and subcategory along with a CI.

[Gusta2307]

What I need is to group all the companies into one category. Is it possible?

[Brian Lancaster]

Can you explain more on what your are trying to achieve? Your screenshot really didn't tell me anything. Why do you want to have a category for company? I'm guessing this is for incident or problem since there is a category and subcategory. If it is for incident why not just set the company field on the incident based on the callers company. 

[Gusta2307]

First of all, I apologize for any confusion earlier. What I’m trying to do is group all the CIs by their type (such as Server, Computer, etc.), but also to group them by companies, as I showed in the previous screenshot.

My goal is to have all these companies as subcategories under a main category called "Companies." In other words, I want "Companies" to be a dropdown list with several subcategories, which will be the companies. Then, each of these companies should have its own dropdown list with the different types of CIs.