After reading this post What is the purpose of 'Arcade Gear Box' anyways?, I have an idea that's the opposite function of "arcade gearbox" and it's a bit, less realistic than, well, realistic gearbox. So when in "Manual Override" shifting mode, you can select any gear not in the gear selection such as forcing the PRND21 Gavril 6-Speed Automatic to select 3rd gear, or in a modern ETK shift to M3 at full stop where the gearbox will auto downshift to M1 in realistic mode.
GreenYellowOwl's transmission pack has a very similar feature where you can custom make a gearbox order (e.g. making something like PRNS1 or PRNDSM21) and also allows you to disable automatic downshifts in automatic and DCT gearboxes, it's not quite there but I do recommend checking it out if you haven't already.
