When a musician improvises, he doesn't do it note by note. Instead, he uses his chops, which are sequences he's practiced hundreds of times. He may be inspired to play a 3-note arpeggio, (which his fingers then do subconsciously, even thought he's fully aware of the arpeggio). So it's a 3-note sequence, even though he only thinks of it as one thing, a unit of music. Based on that arpeggio, he may be inspired to play 3 more arpeggios, each one starting on a note higher, giving a whole ascending series. It's composed of sequences of 4 arpeggios, composed of 12 notes, but he now thinks of it as a larger unit of music. At that point it has truly become planned out in advance, because it may take several seconds to complete the whole thing. After that he may be inspired to play the whole thing again, but this time upside down, starting high and finishing low. Again, it's a predetermined sequence. (For sure, music is usually a lot faster than dance, but it could be a dirge). He's improvising, but just in larger units than single notes.
If I feel inspired to do a giro to the left, there are many to choose from based on the situation - which feet my partner and I are on, whether her next step is inclined forward or backward - so there are many possible giro sequences (chops) that I may choose from. I would subconsciously choose the appropriate way to begin. I would call that improvising, but using predefined sequences. The sequence may be altered in mid-step, or not, but my initial intention was a single unit of dance, composed of a sequence.
I would follow that with something, perhaps a different unit of dance. If I liked those two units together, and how they went with the music, I might decide to repeat them, giving another longer section of dance, planned out into the next few seconds, and fitting into the music nicely. That little repeated sequence would give a structure to my dance, related to the music, that I would enjoy.