Reference designators are arbitrary. Very generally, the first character denotes the type of component (R for resistors, C for capacitors, U or IC for integrated circuits, D for diodes, Q for transistors, Y or X for crystals, J or P for jumpers and/or connectors, F for fuses, K for relays, etc...
The numbers are just as arbitrary. You shouldn't be confused by the number of digits. I typically re-annotate my boards so the designators "flow" according to how they're placed, with 1 being in the upper left and increasing as you move across and down the board. I've tried numbering 1-99 for components on the top and 101-199 for components on the bottom, but over the years my boards sometimes got to be more complex and I opted for just starting at 1 at the top left and letting the numbers just naturally progress as they transitioned to the bottom of the board.
Of course, when you re-annotate based on physical location then the numbers on the schematic are all over the place, but that's fine; trying to find R75 when you see R73 is much, much easier when they're numbered by location. The schematic organization is much easier to observe and doesn't need numeric "locality".