Every audience has an App selector above its filters. It does two jobs at once, and neither is obvious from looking at it: it decides which guests the audience can reach, and it decides which filters you are offered. Set it before you start building, because changing it afterwards can take away filters you are already using.
Where to find it
Open Audiences, then either select + to create an audience or open an existing one. App sits between Categories and Filter, with its own note:
"Specifying an app unlocks more rules you can use. For example you could target people staying in specific rooms."
The options are All Apps (Limited Rules) followed by each app on your account. On a multi-property account that is usually one entry for the group and one for each property. The same app names appear in the App column on the Properties screen, so that screen is the quickest way to see which property each option here refers to. See The Properties screen.
What it does to your audience
Selecting an app limits the audience to that app's guests. Choose a single property and the audience reaches guests of that property only. Choose All Apps (Limited Rules) and it can reach guests across all of them.
This is the part that determines who receives a campaign, so it is worth checking on any audience you inherit rather than built.
What it does to your filters
Selecting a single property adds the room-level filters. Rooms, Room types and Ordered are offered only once a specific app is selected. This is what the note on screen means by unlocking more rules. What each of the three does is covered under The stay itself in Audience filters: reservation.
Selecting an app can also take filters away. The filters describing where a booking came from, such as Channel Manager Name, Channel Segment, Market Segment and Guarantee Type, are not offered for every app. Which of them you see depends on the app you have selected, so a filter available under one app can be absent under another, and absent under All Apps (Limited Rules) while present under a property, or the other way round. These four are documented under Where the booking came from in the same article.
Because of this, All Apps (Limited Rules) is not reliably the widest choice, despite its name. On a multi-property account it can offer more filters than the group-level app and fewer than an individual property.
Once an audience has filters on it, the note gains a second sentence:
"Changing this will remove some filters"
That warning is accurate but not specific. It does not say which filters will go, and a filter you are actively using can disappear when you switch.
Set it first
A new audience does not start on All Apps. It starts on the first app in your list, so a blank audience is already narrowed by a choice you did not make. Decide the App before you add a single filter.
If the filter you need is missing
Check which App is selected. On a new audience it will be an app rather than All Apps.
If you need Rooms, Room types or Ordered, select the specific property. These never appear under All Apps (Limited Rules).
If a channel or market filter is missing, try All Apps (Limited Rules), then each app in turn. The filter may exist under one and not another.
If it appears nowhere, the data behind it is probably not reaching Bookboost. Open Reservations and then Properties to see when each property last received data, which The Properties screen explains how to read.
A filter that is there but empty is a different problem
An empty value picker and a missing filter have different causes. If the filter is in the list but its picker has no values to choose from, the App is not the reason. The filter is telling you that nothing has reached Bookboost for that field yet.
This matters because the two look similar and the fixes are opposite. A missing filter is a question about which App is selected. An empty picker is a question about your data, and switching App will not change it.
What this does not do
It does not tell you which filters you are about to lose. The note warns that some filters will be removed, but not which, and it appears only once filters are present.
It does not leave a record of your reasoning. Nothing on the audience explains why a rule you once used is no longer selectable, so an audience built under one App and later switched to another can quietly lose part of its logic.
It does not replace the Properties filter. Both narrow an audience by property. Use the Properties filter when you want to name properties inside the audience logic, and remember that the App selector is also narrowing it.
What to do next
Once the App is set, Reading the insights panel while building an audience is how you check who the audience actually reaches, because the App you chose decides that total.
The Audiences list screen covers finding and managing audiences once they exist.
See Audiences for how filters combine and an index of where each one is documented.
Getting help
Open Help at the bottom of the left menu and choose Talk to us. Tell us the audience name, exactly which App is selected, and the filter you expected to see, because the answer is almost always specific to that combination.