Hi there,
I’m definitely not a lawyer, but my understanding is that you’re probably fine.
The rights of people accessing your software over a network is the major difference between the GPL and the AGPL. Under the GPL (which includes ERPNext), a person logging into your server would not be entitled to the source. Under the AGPL (which includes most of Frappe’s newer apps), they would.
In either case, that’s only relevant if you’re modifying ERPNext itself. If you’re creating a new app that accesses ERPNext doctypes via APIs, it is my understanding that you can license the software however you like.
Rushabh, Frappe’s CEO, suggests that API usage would not trigger the (A)GPL here: