History is basically a storage where you can store almost anything with the raid. Originally it was planned to use with some AddOns output XML files, but afaik it is not used by anyone (if I remember correctly eqdkp addon can produce XML output files).
It’s really bad idea to overwrite component files. You should never do it. If want to change comething that you can’t make from css, create a folder in you template directory and name it “html”. In this directory create a folder called “com_raidplanner” and create an “event” folder in it. Put your customized default.php file here, and the updates will not affect you.
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