The Splitter must be configured with a Selector to define it’s input - this Selector must me a List/Array type. The Splitter will then iterate over each item in the List and process it individually.
Everything beyond the Splitter will be executed for each item in the List - so consider this in your flows. For example, if you have a Splitter with a List of 10 items, and then a Slack Action, you will send 10 Slack messages.
The config for Splitter consists of
Item Event Type - which currently is defined to a default value for you - you can mostly leave this alone.
Then we have
Item Content Type Override - as it’s name suggests, can be used to override the output content type - otherwise it will be inherited from the Selector you’ve chosen - for the most part this can be left blank.
Finally there is the
Selector - this is the important part, and is used to define the List/Array of items to be processed.
For example - "Contacts" is an "array" type