A record locator is an alphanumeric reference number used by airlines and travel agencies to identify a booking.
It usually consists of between 5 and 7 letters or numerals, thus allowing a sufficient number of unique combinations. Airline systems only work in upper case, but this does not unduly restrict the possible combinations. Some systems, but not all, avoid the letters I and O for their visual similarity to the numbers 1 and 0.
Each airline will allocate its own record locator, and if the flight is booked by a travel agency system, then the agency will also have a different locator.