Wherever an entity (fDocs, fPanels, fSheets or Menu) must be selected, click on the entry box or the icon next to it to open the Entity Picker.
Only entities relevant to the context where you are picking the entity will be shown.
You can also filter the list using a text search, selecting only those you have been involved with or by choosing index labels (if any have been assigned to the entities shown).
Click on an entity to return it to the entry.