The PST file — or PST files; you can use multiple PST files with one copy of Outlook simultaneously — holds emails, contacts, notes, the to-do list, calendars and other Outlook data.
The use of a single database for all data makes PST files grow rapidly and to significant sizes. Even when the PST file size limit is not yet reached, it is recommended you use multiple smaller PST files to ward off the danger of file corruption and slow access times.

