Category field signification and usage

Hello,
I do not understand what the category field stand for.

Does it serve to divide the project into several stages ? In this case the category must be attached to the project and not the reverse.
Does it serve to define several typical steps that are used in several projects?

I thank you specify what you had in mind when you created it and how to use it.

May be it could be fine to add a field with the theorical end date (or amount of time) of the project. In that way i kwow if i'm late or not and if the charged costs


REQUEST :

It might be interesting to add a field with the theoretical end date of the project to see if deadlines are met or not and if the cost charged is correct.

Also, in the case of a maintenance sum, it would be interesting to have a field value that changes dynamically based on time spent on the project.

Let me explain: the customer pays € 250 for maintenance costs. Every time I work on the project the amount of time worked is removed on the € 250 and when the field is 0 I know I need to start a new billing.

Brunov99