Most airlines confirmation numbers (or record locator) are sequential, not random. Starting with a letter but including both numbers and letters. But do not include the letter I or O since they could be easily confused with 1 and zero. 10 digits and 24 letters allows for a very large number of unique 6-character confirmation numbers before they would need to be reused.

HXEL28
HXEL29
HXEL2A
HXEL2B
...
HXEL2H
HXEL2J
...
HXEM1B
HXEM1C

When airlines and GDS systems send messages to each other, they always identify the airline that owns the booking:

HDQAA A23456 The booking with confirmation number A23456 is owned by American Airlines and was stored by the AA headquarters location.
MUC1A A12345 The booking with confirmation number A12345 is owned by Amadeus and was stored by the Munich computer center.