A call history file contains call signs and exchange information that can be used with the N1MM Logger+, and possibly other contest logging software.
When you type a call sign and leave that field, the logger automatically fills the exchange fields (just one – the county – in the Virginia QSO Party). You type nothing if the prefilled exchange is correct.
I wrote a program to read VAQP logs, and extract call signs and counties (states, if outside Virginia). The call history file contains 1,494 call signs extracted from six 2017 VAQP logs.