Actually, all of the variables are custom variables. They can be whatever you want. The objective function, which you write, gets passed a table of values for the optimizableVariables you have defined, and you write the objective function to use them however you want.
The section 'Objective Function for Optimization' shows how an objective function can be defined. The variable 'optVars' inside the objective function will be a 1-row table with one column for each of the optimizableVariables you've defined. You can see the variables being used inside that function. Look for 'optVars.NetworkDepth', 'optVars.InitialLearnRate', 'optVars.Momentum', etc. You could create as many other variables as you want and access them inside your objective function in the same way.
This page explains a bit more about objective functions and optimizableVariables: https://www.mathworks.com/help/stats/bayesian-optimization-objective-functions.html
