Batch offers two solutions in case you need to target specific user IDs:
Send real-time notifications using the Transactional API
The Transactional API is made for 1-to-1 or 1-to-few interactions. It allows you to send push notifications to a specific list of IDs (push tokens, Advertising IDs or Custom User IDs) based on events in your back-end.
Use it to send action-oriented or time-sensitive push notifications.
Action-oriented notifications: New message, friend request, reached credit limit, user's turn in a game, etc.
Time-sensitive notifications: Delivered order, delayed flight, canceled train, etc.
Create and target segments using Batch's Custom Audiences
Custom audiences allow you to upload static segments exported from your userbase (e.g. top 500 buyers, etc) or created by third-party tools. You can easily retarget these custom audiences with a push or an In-App campaign from the dashboard.
You can easily create a custom audience by uploading a .txt or .csv file from the dashboard Settings > "Audiences" tab:
You can also create and update audiences remotely, using the Custom Audience API.