What are Auxiliary qubits?
This term is mentioned in Codebook exercise I.13 as below:
Hello @Manu_Chaudhary, there are good answers on StackExchange: terminology - What counts as an "ancilla" qubit? - Quantum Computing Stack Exchange
Hey @Manu_Chaudhary , ‘auxiliary’ literally just means that they’re helping, we’re using them for something, but we don’t really care about them. They’re just there to help us get to the result we want — we call them auxiliary because we usually have to add some to our set of qubits in order to perform the calculation. 
Thank you @Ivana_at_Xanadu for response.
Suppose, I am using a Control gate with 3 control signals and 1 target signal, then:
total qubits =4
auxiliary =3
main qubit =3
Is i am correct?
Please explain with example. That would be a great help.
You can think of ‘auxiliary’ to mean ‘control’ here. So, 1 target, 3 little helper qubits (in a known state!) that just help you get what you want on the target. It’s not a make-or-break way of naming things, so you don’t have to worry too much about it, I think. 