In a Mainframe computer, there would be thousands of files(datasets). It is obvious, to name each Dataset differently. Mainframes support very large dataset names. In Mainframes, a dataset name can be 44 characters long in this format :
XXXXXXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX.XXXXXXXX
You can form a dataset name by taking groups of upto 8 characters. Each of the group must start with a capital letter(alphabet).
Generally, it is a good practice to give meaningful names to your dataset. For example, if you are storing Employees data in a file, you can name it as EMPLOYEE.DATA
The MVS Operating System keeps track of groups of datasets by referring to their names, which are called Qualifiers. The first part of the dataset name is called High-level Qualifier(HLQ).
Generally, when you access or log-in to Mainframes, you are given a USER-ID, just as on Windows PC, you need a userid to login. Most professionals or software engineers who work in Mainframe Projects would have a TSO user-ID. When you use a TSO-id, a special requirement applies to most datasets(files) that belong to you.
Suppose your TSO-id is AGY0157, then all your datasets should have the High-level qualifier AGY0157. Thus, the files that belong to you should start with AGY0157. For example, the name of Employees file would be
AGY0157.EMPLOYEE.DATA
You must understand that, mostly in projects, you can identify the files that belong to you, your application or your system by seeing the High-level Qualifier.
In fact, Mainframes are also installed with Security Software(like RACF), that controls access to files by seeing the High-Level Qualifiers. Thus, although you can see your buddy’s files on a Mainframe, you cannot make any changes to it.