Usage of this parameter is dependant on function definition id and ItemType :

• id: 3 (Rectangles) ItemType: Eplan::EplApi::Base::Enums::ItemType::DrillingRectRounded - diameter
• id: 3 (Rectangles) ItemType: Eplan::EplApi::Base::Enums::ItemType::DrillingRectChamfer - chamfer length

In other cases this parameter is not used.

Reference

Drilling Class
Drilling Members