In the context of transcription, net line billing counts a line as the total number of characters divided by a defined net length in characters - here at Hypertype we typically use 65 as a defined net length. Assuming the net length is 65 characters, if there were 650 characters on a page there will be 10 lines on the page.
At Hypertype we use net line billing, we only count partial lines for what they actually contain. For example 7 characters would be billed as 7/65 of a line, instead of charging for an entire line!