Old style task pane addins (Office 2013) get embedded in the open document at the time of add-in installation and persist in it. The task pane can be closed but when the document is reopened the task pane re-opens too. If a user that does not have the add-in installed and tries to open the document he will be prompted to install the add-in.
With newer task pane add-ins (Office 2016) the task pane can either be opened from an Office UI Ribbon button or be auto-opened on document launch, for properly tagged documents, via the new auto-open feature, see: https://docs.microsoft.com/en-us/office/dev/add-ins/design/automatically-open-a-task-pane-with-a-document
With newer task pane add-ins (Office 2016) the task pane can either be opened from an Office UI Ribbon button or be auto-opened on document launch, for properly tagged documents, via the new auto-open feature, see: https://docs.microsoft.com/en-us/office/dev/add-ins/design/automatically-open-a-task-pane-with-a-document
