There are 2 schools of thought on Online publishing and Digital publishing. One school believes they can be used interchangeably, the other believes they are not the same.
the major difference is that digital publication (e.g. PDF document) may not be accessible online (but it still can be read e.g. on kindle).
Some assume that an online publication is publicly available, but that does not have to be true. Sometimes you have to pay to access some online publications.
Every online publishing is digital publishing, but every digital publishing may not necessarily be online. Still more, every online publishing is not necessarily open access, but every open access publication is generally online and none of them can be used interchangeable............