Redmine

Hello, I want to extend the functionality of the context menu of a Redmine plugin. I found the context menu view I want to modify. However I feel lost. I am new to Redmine api. I want to be able to select some items and make a function to only show these items. My question is how to get the selected items. Are they stored in the session or..? Any documentation you can point me to regarding context menu will be of great help as well. Thank you for your time!