Q: Why are vertical support cleats being applied even though I changed the cleat rule to Horizontal Only?
A support cleat will be applied if a seam or splice exists.
If your dimensions show for the panel that a seam or splice is required* then a cleat must cover that seam. If the length dimension is greater than the length of the panelboard material being applied then you'll have a horizontal splice. If the height dimension is greater than the material being applied then your panel will include a vertical splice.
So if a seam exists, Crate Pro 6 will add material to cover that seam even if you select a rule for H1 -Horizontal Only or V1 -Vertical Only.
If you select the Horizontal ONLY or Vertical ONLY rule, only the required quantity of cleats will be added to cover the seams. These cleats will not be the dominant support cleats.
Versus if you select a H6 - Horizontal FIRST or V6 -Vertical FIRST rule, Crate Pro 6 which will apply the support cleat spacing rules set in Preferences -> Construction -> Rules & Other for the style family being used.