I spent over an hour on live chat today to find out that custom roles and access set up in the desktop version don't carry over to the browser version, even though they appear on the list of options after clicking on the user's name. So there's no way to give partial access to parts of sales or purchases, or any other area. It's all or nothing. And if someone needs partial access to the file, then you basically have to set them up twice. Once in the desktop software and again in the browser.
I couldn't find any documentation anywhere to say this, and the live chat person couldn't tell me where to find it either.