Glad to hear that you love your new SM200B. That’s great!
Regarding preselectors, check out section 3.3 of the product manual linked below for the full description.
https://relatedcontentdemo.kinsta.cloud/sigdownloads/SM200A/SM200-User-Manual.pdf
Above 645MHz, the preselector filters are always on, since we have enough filter overlap to measure at any of those frequencies with any IF bandwidth. The narrower filters below 645MHz are the ones that are controlled with the preselector control. For sweeps, enabling the preselector will slow down the sweep for the benefit of the extra filtering. For I/Q streaming and real-time measurement applications, enabling the preselector means we can only enable a single filter. In these situations, you can refer to the filter list in the above manual to see which filter is used at specific frequencies.
I hope this helps. Let us know if you have further questions.
Andrew