SPLIT is used to split a string into pieces by a separator character.
If you get a delimited list of customer_id like "1234,1235,1256" you can split it by SPLIT ({field},',') into an array that contains one entry per customer_id
The error message that you got is that formulas cannot return array values. if you want to calculate it and return it elsewhere add a "" after the calculation. the formula returns the result of the last operation.
...
myNumbers := Split({AField},',');
""
will make it executable.
