It's not very restrictive. However, rules are different from conventions. Java naming conventions for variables, methods and reference types are where things get a little more complicated. For example ...

A random variable is one whose value is unknown or a function that assigns values to each of an experiment’s outcomes. A random variable can be discrete or continuous.

Different types of data need to be presented in appropriate ways. Learn to present types of data with BBC Bitesize. For students between the ages of 11 and 14.