The Aggregate Field evaluates on the client dynamically and does not save to record data, this means that you can have a summary value of a number field that can be used in templates and dashboards that doesn't get saved to the record. The benefit of something like this is that it can greatly cut down on the number of expressions that are being run on your apps and the time to sync data. Such App Level Field Types can only be referenced within App Level based Templates.
To configure an Image Action Button, you first need to be in the 'Fields' section where you will then be able to click 'Add Field' in the bottom left of the screen.
From here, either search for 'Aggregate' in the top-right box or scroll down to the relevant tile, confirming one of these two will consequently present you with the Field configuration options below:
Label: This is the text that will appear above the field in workspace, this is best to be a short description of what the field will be displaying so the user knows the context of what they are looking at.
Identifier: This should reflect the label. The identifier is a system ID for referencing a Field. This ID must be unique per Field and must contain only letters and numbers starting with a letter. It must also be a minimum of three characters.
Type: The sort of Field that is being configured - With some types this can be edited in the future providing the data type is cross-compatible.
Reference Field: The Reference Field is the field used for the summaries to calculate on, for example, the sum of a cost field, these can only be number-related fields. The list will only show the field in the app that can be used.
Summary Expression: The Summary Expression option determines which function to use during the aggregation process:
Filter: The Aggregate Filter is used to narrow your target data, for example, you may only want to sum the data of deals that status is red. This feature will be coming soon