See discussion at https://groups.google.com/d/msgid/tiddlywiki/526754d5-8786-49cd-aaa1-c77311670116%40googlegroups.com:
The history here is that in v5.1.14 we added an "actions" attribute to the checkbox widget that specified an action string to be specified whenever the state of the checkbox changed. The same action string is executed regardless of whether the checkbox was checked or unchecked.
Then in v5.1.16 we merged https://github.com/Jermolene/TiddlyWiki5/pull/3182 which added the "uncheckactions" attribute. The difference between the two is that the uncheckactions are only executed if the checkbox is unchecked. The main "actions" attribute is executed before any actions in "uncheckactions", which makes it possible to arrange things so that the "actions" attribute sets things to the checked state while the "uncheckactions" attribute overwrites those changes to set things to the unchecked state.
In retrospect, it would have made more sense to have also added a "checkactions" attribute that was executed only if the checkbox was checked. I'll investigate adding it for v5.1.20.
