Usually Outlook loads all add-ins in their application domains, to isolate them from Outlook and from each other. In that way, unstable add-in will be prevented to influence on Outlook or the other add-ins. If some add-in hung, Outlook will not load it next time it starts.
But, control hosted in custom FHP work in the same application domain as Outlook, which can be risky because any instability in add-in can endanger the whole Outlook. FHP is an acronym for Folder Home Page - an HTML page that may be displayed when user clicks on the folder icon to represent its content. That page may include any other user control, which is a usual way for a developers to embed a new control in Outlook.