A technical sketch can explain a detail too blurred or too small to be clear in the costume drawing. with a line drawing and notes about measurements etc., you can specify such things as the size and kind of cuffs required, the shape and decoration of a skirt flounce, of the construction of a cravat. Do not forget to tell the makers about hidden details such as pocket positions.
Notes can indicate any special requirements you may have about the use of fabrics in juxtaposition or layers.
Take care to show the placing of braids and other trimmings on the costume. It is helpful to give measurements wherever possible.