This might give you some insight, I don't know. Still new in everything here.
WHAT IS THE DIFFERENCE BETWEEN A WORKING CELL BANK (WCB) AND A MASTER CELL BANK (MCB)
A Master Cell Bank (MCB) represents cells "frozen in time". At some point in history, a type of cell was seen as desirable for study, cloned, processed, and stored in extremely cold conditions, effectively freezing exact copies of that cell type. The storing process is usually done using small cryovials stored in liquid nitrogen (LN2), using a technique called cryopreservation.
A Working Cell Bank (WCB) is a stock of cells that have been or are being "thawed out", and grown in a cell culture (effectively cloned), taken from a sample of cells in the MCB that is considered pure. Using a WCB, one can create any number of copies of a specific cell line. Note that the sample of cells in the MCB are not physical locations like Fort Knox where a large amount of cells are kept under guard, but rather terms used to describe the work of many scientists and organizations world-wide who have established their own independent cell banks.
Different techniques are used to determine the relative purity of a cell culture, for example, there are ways to test its purity, identity, and genetic stability.
Please not that not all Contract Research Organizations (CROs) follow the same guidelines, or perform the same type of tests.