Via an in-app purchase you can enable the Tagging expansion.
Tagging adds the ability to add labels to Games, Locations and Players, and filter the lists in the app based on those labels, called tags. It’s now possible to create your own Game filters and save them, based on a number of different criteria including tags. For the games in a saved filter a combined statistics page can be viewed. Also, the Insights screen can be filtered based on these Game filters.
Game Tags can optionally be kept in sync with BoardGameGeek, manually or automatically. Tags and saved filters are synced via Cloud sync as well.
If you already have a lot of tags on BoardGameGeek which you want to continue using in the app, the best first step after purchasing Tagging is to first sync your tags with BGG, so you can immediately start using them.
A video of tagging in action is added at the bottom of this page.
New features
Add tags to Games, Players and Locations
When viewing a Game or Player, or when editing a Location, you can now add Tags. Press the Add… button to start adding tags. 
Type the name of the Tag you want to add in the field on top of the dialog and press Go on the keyboard to add the Tag. Tap Done save the changes.
The selected tags will be listed on the overview screen as well as in lists. For the Games list, the tags are only displayed when the view option to view Large images is turned on.
Using multiple-select in the Games list you can also apply tags to all selected games at once.
Filtering Players and Locations on Tags
The Players and Locations lists now have a dropdown on top from which you can select any of the created tags. The list will be filtered on the selected tag.
The new Filter Games menu
The games list already had a dropdown on top where your collection can be filtered on predefined filters and collection statuses. This menu has undergone some changes.
There is now an Edit Menu… button on the top right, with which you can select what you want to view in the main dropdown, and what you want to move to submenus.
A separate Tags submenu has been added. If you select one of your created tags from this submenu, the list of Games is filtered on that specific tag. In the submenu you can tap the star icon to favorite tags, so they can appear in the main menu, or as a separate submenu for easy access.
Creating Saved filters
Selections you make in the Custom filter can now be saved as a Saved filter. Press the Save… button to give a name to the new filter, and it will appear in the game filter menu.
When viewing a saved filter, you’ll see an option to Edit that filter, where you can change the criteria, rename or delete it.
It will also show the Stats > button, which displays a combined overview of games resulting from the current filter. This button is also available in the Custom filter.
Creating Advanced filters
By pressing the Advanced… button in the Custom filter you will be taken to the Advanced filter screen, where you can add any combination of filter criteria.
You can add any number of new rules to the filter, with the dropdown on the top
left. All rules must apply for games to be in the results of the filter. The current number of resulting games can be seen on the top right.
Swipe right to left over a rule to delete it from the filter.
The Custom filter’s Advanced button will show a checkmark if advanced rules are applied. You can always completely reset the filter with the Reset button.
Read more about the Advanced filter.
Filtering Insights on game filters
A new filter on the top right of the Insights screen let you filter the stats on any of the standard filters, tags or a saved filter. The selected filter is combined with other filters from the Deep statistics expansion, if you have those activated.
Syncing Game tags with BoardGameGeek
The Tags you assign to games can be synced with BGG. In the Settings, under BoardGameGeek you can find a new option Tag sync settings. In there, you can sync tags manually with the Sync tags now button, or turn on Auto-sync.
As tags on BGG cannot have spaces in them, any spaces you used in tags in the app will be converted to dashes. BGG tags also can’t contain any emoji’s. For that reason tags with emoji’s in BG Stats are skipped completely during sync.
The Sync tags now button will scan for changes between the app and BGG and display the changes for you to confirm, before continuing the sync.
Turning Auto-sync will sync tags for a game immediately as soon as you change them, and check BGG for changes as soon as you view a game’s overview screen. On startup of the app a full scan will be done as well, and automatically synced if there are less than 10 changes found. If there are 10 ore more, a confirmation will be requested before a sync is done.