This topic has come up for me at this point in many horn lessons over the years. The short answer is that your horn may not have one even though you may honestly believe that it does.
Among horns I see most commonly standard model Holton horns have a small slide that is a separate B-flat horn tuning slide tucked up next to the change valve, but most Geyer type horns with the change valve on the other side of the valve section do not. And a Conn 8D, seen in the photo, also does not.
Stepping back, if you take your horn and follow the tubing from the mouthpiece, the first tuning slide you come to will be your main slide. Then besides the valve slides on the typical double horn you will have one to three more slides. If it is a Geyer style horn you have one more on the back and it only impacts the F side. A horn built like a Conn 8D has two slides in addition to the main slide. Those slides BOTH are F horn tuning slides.
The slide with the big arrow in the illustration is an F horn tuning slide. Over the years however I have ran into many players who thought it to be a B-flat horn slide. I guess because it is on the front.
If you are in doubt of the function of this or any other slide just take the slide out and try to play the horn. If it works on the B-flat side with the slide out but not on the F side, then the missing slide only impacts the F horn.
Then there is the related question of what do you do if your horn is sharp on the B-flat horn but you don’t have a B-flat horn tuning slide? The answer is you need to think the topic backwards; the F side is low in relation to the B-flat side. In other words the F side is pulled out too much. Bring it up to match the B-flat horn then bring the whole horn down with the main slide. Intonation works a lot better if your horn is in tune with itself.