Example of Dimensionality Mismatch
In the following example, the unitPrice variable cannot be linked to num1 because the unitPrice variable has a dimensionality of 1 (String (0) + Document List (1) = 1) and num1 has a dimension of 0.
unitPrice cannot be linked to num1 because of dimensionality differences
To solve this, you can either:
Change the service invoked by the transformer to accept arrays as data, or
Create a flow service in which a LOOP step loops over the array variable. Then, (in the same flow service) invoke the service you originally wanted to use as a transformer, and make that INVOKE step a child of the LOOP. Finally, insert the resulting flow service as a transformer in the MAP.
Of the two options, changing the service to accept arrays as data results in faster execution of flow services.