A contract specific limit exists, that restricts the number of trackable expressions per dimension (e.g. page, event, ad media, ...). For example: How many different pages can be tracked in total?
When the limit is reached, no additional expressions (e.g. new pages) can be tracked. Instead, these are grouped as "webtrekk_fallback".
Therefore, Mapp provides the possibility to clean the data. Please refer to the Support for activating this.