AUs are plots where you need to change the canon for them to fit.
The difference between Serious and Crack AU depends on the tone of the convo, and is left up to the archivist's appreciation. Sometimes, an AU has the potential to be serious, but only jokes and memery were generated during the convo; if that's the case, it's categorised as a Crack AU until something more consistent is made of it.
Crack AUs are much more loose on the "Creator" part, since they tend to be the result of server-wide brainstormings.
Important: These AUs being very messy and only for fun, they don't have character tags.