What is graining angle ? What is the purpose of if ?
Specifically in plastic design what is use of it
Normally when you grind a sheet metal part you get a direction of the your grinding and the grain defines how "smooth" your surface is.
When you have more than one part and you want to use a previous ground sheet metal plate than the graining angle is essential for nesting. You can cut your parts from the plate only in two directions and you have to place your parts in an approbriate order. Essential is this angle when you use a nesting program.
e. g. Sheet 1 ISO 9445-1.4301K240-1500x3000... a sheet metal plate with 1mm thickness and dimension 1500mm by 3000mm, material quality 1.4301, grade of grain K240. The grain direction is parallel to the longer dimension.
You have to put your wanted blanks according this direction. The graining angle is the angle you must rotate your part to align your wanted ground direction on the plate.
Whats the special use in plastic design - I don't know
I've never seen this term "grain direction" in CATIA. Is there a particular workbench or app that requires this as input? Understanding the context of your question may help us provide an answer for you.