A hack series must contain at least two published ROM hacks that are canonically part of the same series which have both been worked on by the original author(s). This can include spin-offs, sequels, prequels, or demos.
There are many stand-alone hacks that have sequels planned by the original author(s); in the instance of these stand-alone hacks, they will need at least one more published instalment to be considered part of a hack series- a "planned sequel" isn't enough for a stand-alone hack to become a series.
