For the most part the Drexel MSMA 6 Plus and the MSMA 6.6 are close to the same herbicide just different manufacturers and slightly different percentages of active ingredient between the two.
They are both labeled for use in similar areas: Cotton, Golf Courses, Sod Farms and Highway Right-of-ways. You can see all the feature differences for comparison Here.
Drexel MSMA 6 Plus application rates vary depending on the target area you are applying on.
Please refer to the product label for mix rates based on the application site.