zp is a distance between the bolt line and the weld line in the single plate. In the case of extended plates (beam to column web connection with stiffener), this is the distance between the bolt line and the support outer edge.
So in this specific case, the connection is with extended plates ('Beam end inside support flanges' option is not checked), so zp value in the input is measured as the distance between the first bolt line and column flange end. The value used in the design is equal to a zp value entered by the user (55 mm) + a value from the column web till the end of the flange (147.5 mm).
So in this specific case, the connection is with extended plates ('Beam end inside support flanges' option is not checked), so zp value in the input is measured as the distance between the first bolt line and column flange end. The value used in the design is equal to a zp value entered by the user (55 mm) + a value from the column web till the end of the flange (147.5 mm).