For fun, I pulled the directives in the USDF dressage tests. Fluency does not show up at training or 1st level. In 1-3, for the leg yield, the directives indicate: Regularity and quality of trot; consistent tempo; alignment; balance and flow.
Fluency shows up for the first time in 2-2, in the half turn on haunches: Quality of shortened walk strides; tempo and regularity; activity of hind legs; bend and fluency in turn. Same in 2-3.
In 3-1, it shows up in the 10m half circle to half passes: Shape and size of half circle; alignment, bend, fluency and crossing of legs; engagement and self-carriage, and again in the turn on haunches: Quality of shortened walk strides; tempo and regularity; activity of hind legs; bend and fluency in turn. Lastly, in the changes: Clear, balanced, fluent, straight flying change; engagement and self-carriage. In 3-2, you have it in the half turn on haunches, then the half pass says this: Alignment and bend while moving fluently forward and sideways; engagement and self-carriage. Also shows in the changes. It’s in the directives for 3-3 on the half passes (both trot and canter), turn on haunches, and changes.
In 4-1, 4-2, and 4-3, it’s in the directives for half passes at trot and canter, walk pirouettes, walk-canter transition, and changes (and there’s a lot of changes in 4-3).