Swing style. During the double bass improvisation , the drums must not continue to swing in the normal accompaniment but only be limited to some sounds in response to the double bass. Normally I have heard this done. How can it be done with BIAB and Realdrums?

I'm assuming you have a complete song part of which is a bass solo. is the bass a BIAB track or are you playing it live? BIAB doesn't do improvisations unless you count regenerating tracks. (Or did you mean a bass solo generated by BIAB? - if so see later)

If you have a complete song and you want to improvise a solo at a set point you will still have to fix what the drums play - BIAB and RealBand can't 'respond' like a real drummer.

My first response is that it would be easier using RealBand and midi drums. Unfold the song so you get a long file with a single chorus. Save it and open in RealBand.

Go the bass solo part of the song. delete the real drums for that part only.

then decide what you want to hear from the drums. Choose a new midi track and using the drum grid editor 'fill' each bar or bars with what you want.

If you meant a BIAB generated solo, i think you should generate one you like then freeze it and use that to decide where you want the drums to play in relation to that solo.

I would concentrate on obtaining the drum licks that will be played back during the bass improvisation first. You likely have an idea how many bars you want the improve to go so develop the drums for those bars first. The drums for "X" number of bars may originate from RealDrums, midi drums or loops; it really doesn't matter as much as ending up with "X" bars of drums playing licks that you want. For now lets say "X" = 8 bars and the song is 40 bars long.

Record the 8 bar drum licks to audio. If you want to, repeat the drum licks recording 5 times so you end up with an audio file that is 40 bars of drum licks. Remember these drum licks are audio so they reside on the audio channel in Band-in-a-Box's mixer. MUTE THE AUDIO TRACK.

Now work on building the rest of the song. When it comes time for the bass improve, mute the drum track and return-to-normal (unmute) the audio track that contains the drum licks. Once the bass improve has completed, mute the audio track and return-to-normal the drum track.

Why did I suggest having the drum licks occupy all 40 bars of audio? That way you can move the position of the improve within the song and you already have the drum licks. All you have to do is change the mute and unmute locations for the drums and audio tracks.

maybe i'm wrong Jim but i think the OP wanted a specific drum response to the bass and i can't see any way of doing that apart from midi.
'some sounds in response to the double bass' is a bit difficult to quantify. maybe cubalibre can be a bit more specific. i'm still not sure if we're talking about a backing track over part of which he will be playing a bass improvisation live, or whether he means a bass solo generated by BIAB where he wants to tailor specific sounds to respond to the bass. maybe recording midi drums by playing on a midi keyboard/controller could be the answer.

'some sounds in response to the double bass' - if he means what i think he means (!) would be difficult to create by generating Realdrums

Bob I found the original post ambiguous so I guessed at the intent behind the post. Like you I hope Cubalibre will clarify what he wants to do.

Two musicians can not interact when one is a prerecorded audio track. I guessed which musician is live and which is prerecorded. I certainly can be wrong.

I apologize if not, I replied immediately ... but I was busy for work. Here is an experiment with a young double bass player (18 years old). The sound of the double bass is a bit stopped is the first test we made to record. If you listen when the bass makes its impro the biab realdrum obviously continues its run and this is not very nice.

FLY ME TO THE MOON (smart working test)