what's the 'regular' type for font_add()

[Lin Zhang 张林]

Hello, I get an error as follows:
font_add('NSimSun', "D:/Desktop/StartR/NSimSun.ttf")
Error in check_font_path(regular, "regular") :
font file not found for 'regular' type

I don't know what's the 'regular' type for font_add()
Thank you
NSimSun.zip