请教这段关于图片的投稿要求是啥意思?

As a general rule the lettering on the artwork should have a finished, printed size of 7 pt for normal text and not smaller than 6 pt for subscript and superscript characters. Smaller lettering will yield hardly legible text. This is a rule-of-thumb rather than a strict rule. There are instances where other factors in the artwork. For example, tints and shadings dictate a finished size of perhaps 10 pt

我不明白lettering, pt 和subscript and superscript 以及tints and shadings 到底指的是甚磨? 谢谢!