Mine is always first, so I can utilize the Resonance feature with my single coils. I will note that I am not a big pedal user, so most of the time if being used, it will be the only pedal in front of my amp. I definitely use it to get a little "more" when needed.
It is true bypass.
I love mine, but temper that with I have not utilized a lot of other Boosts.
If you found yours used, without the manual, the manual is available here. It is text and over a few posts.
https://forum.seymourduncan.com/forum/the-vault/60370-sd-booster-manual?p=840265#post840265