The seams down the front are graded so that
the widest layer will lay next to the portion that
shows on the outside of the garment (Fig. 19). The
lapel wi II be graded so that the widest area wi II lay
next to the facing. At the point where the facing
reverses from the outside to the underside, clip into
the seam and reverse the grading. This will keep the
widest seam allowance on the top and prevent a
ridge from showing on the right side. Usually 3/8"
for the wider seam allowance and 1 /4" for the
narrower are sufficient. Press seams open, then to
one side. Those in the lapel area will turn toward
the garment, the portion below the lapel will turn
toward the facing
https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=5...
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WHY TRIM AND GRADE SEAM ALLOWANCES?
You can sew a lovely neat seam and press it perfectly, but if you don't trim and grade those seams where necessary you will be left with a bulky, bumpy appearance on the right side of the garment. Trimming refers to reducing a seam allowance to prevent bulk. Grading is used in an enclosed seam to reduce the bulk created by the layered seam allowances. Trim, grade, and press in the right places and your seams will look as smooth as silk.
https://fancycollective.com/blogs/sew-easy/how-to-trim-and-g...
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Grading
Increasing and or decreasing a base size pattern to fit various sizes. To reduce layers of seam allowance to reduce bulk when sewing a seam such as around the collar/neckline of a coat.
https://www.universityoffashion.com/resources/terminology/