ADF Debug pipeline : Use activity runtime

Question

What is the difference between 'Use data flow debug session' and 'Use activity runtime' in debug in Azure Data Factory?

I have a pipeline with a Lookup activity followed by a ForEach activity.

A lookup file is used to store parameters, and this file is passed to a ForEach activity.

The ForEach activity consists of another pipeline which has a dataflow. The compute size and core count are passed as parameters from the Look up file. The core count is defined as an integer value.

• On running the debug with 'Use data flow debug session' option, the pipelines runs successfully. These are the compute parameter values passed as input to the data flow : "compute": {

      "coreCount": "16",
    
    
      "computeType": "'General'"
  

  },
• Input to the inner pipeline [which contains the dataflow] :

      "p_compute_size": "'custom'",
    
    
      "p_compute_type": "'General'",
    
    
      "p_core_count": "16"
    ```However, the other option fails.
  
  

• On running the debug with 'Use activity runtime', ForEach activity fails with the error : 'Failure type - User configuration issue'. The dataflow fails with the error : The request failed with status code '"BadRequest"'. The parameter passed is the same :

     "compute": {
    
    
        "coreCount": "16",
    
    
        "computeType": "'General'"
    
    
    },
    ```- Input to the inner pipeline [which contains the dataflow] :  
  
    ```python
      "p_compute_size": "'custom'",
    
    
      "p_compute_type": "'General'",
    
    
      "p_core_count": "16"
    ```This is how it is defined in the Lookup file :
  
  

		"computeSize": "custom",

		"computeType": "General",

		"coreCount": 16
```I don't understand why one option runs, but the other fails even with the exact value for parameters.