Hi,
The only article i could find in Microsoft Site about Windows 10 Backup does not specify what "participating apps" means. Here is the article: Back up your Windows 10 Mobile device
If this helps the best explanation about what " participating apps " is this: Device backup includes your app list only. App data may be backed up if the app developer elected to include app data in in backup. These are the apps that participate in app data backup. Device backup includes phone settings, start page layout, installed app list and data from participating apps. Personal data is not part of phone back up, documents. pictures, videos and music.
That said, before you Reset your phone at least make backups of all apps that on them self’s offer that option, like for example WhatsApp.