A fitting rationale is a “formula” so to speak to decide how compression ratios of gains are applied throughout the frequency range based on your hearing loss to give you the best hearing amplification. Many HA mfgs use their own proprietary fitting rationale that they think will work best for their brand of HAs. But there are also fitting rationales that had been instituted as industry standards to be used for any brand of hearing aids.
Oticon uses their proprietary fitting rational called VAC+ on the OPN line of hearing aids (and probably the previous lines as well). That is what is in your default program, and all the other built-in programs that you choose from their menu.
But, they do offer and make available other industry standard fitting rationales as well, like the NAL-NL1, NAL-Nl2, DSL v5.0 Adult, DSL v5.0 Pediatric.
I find the VAC+ to sound the most natural compared to the other rationales. But the DSL v5.0 Adult rationale and the NAL-NL1 rationale sound a bit brighter. Although this makes it not as natural sounding to me, this brightness seems to help me understand speech better in more rare situations where I find speech clarity challenging even when using the VAC+ more natural sounding Oticon rationale.
So you can opt to have your audi dedicate the remaining 3 programs that the OPN has available to 3 of these standard rationales, just to compare and see if you’d prefer any of them over the VAC+ in the default program or not. This will allow you to do A/B/C/D comparison between the different rationales. Once you settle on your preference, you can keep the ones you like more and remove the ones you don’t care for afterward.