This sheet music was obviously made in conjunction with other references to strumming and picking patterns on a guitar instruction site. What site, who knows. It would take some googling and research and you might could find something through the information at the bottom of the sheet music. But there is no "strum pattern 6" in music notation.
I would go further to say that "strumming pattern 6" is a pattern made by someone that "works" with this song. I doubt the transcriber made this special strum pattern 6 JUST for this song. Also, I would venture to say that Tracy Chapman did not adhere to a "pattern" measure after measure, so how long would such a strum pattern be accurate? One measure? IF you are going to spend time and energy and devote brain cells to learning a strum pattern, you would be better served learning to play along with the song. As you said, you can hear it.
As far as standard notation, you are rarely if ever going to get an accurate strumming representation of a song from sheet music or computer generated software. There are just too many variables... up, down, what strings, speed of the strum across the strings (not simultaneous), volume dynamics, etc. All that for each and every strum, and that is not even talking about timing variations between strums. There is a wavy line symbol in music notation that denotes that stacked notes are strummed and not simultaneous played but that doesn't tell you the intricacies. To notate all that would be a crazy endeavor.
I use sheet music or youtube or guitar pro or whatever for quickly getting the basic solution or for structure but ALWAYS refer to the song. Inevitably, you will hear something different or missing.