If I have understood correctly, you want to classify the data for your Marketing Channels. You can do so by creating the classifications for the Marketing Channel. Below is the documentation link using which you can create the classification structure.
Once the classification structure is created, you can create a classification rule set in the Classification Rule Builder user interface to classify the values based on your rules. For more information on classification Rule Builder, you can refer to the below documentation link.
Also, I would like to inform you that if you are looking at the Marketing Channel data then use Marketing Channel as the dimension & use it along with the Marketing Channel Instances metric. In case if you want to break down the Marketing Channel with any other dimension then set that dimension in the 'Set the Channel's value to' from the Marketing Channel Processing Rule user interface as shown below.
After setting the channel value you can breakdown your Marketing Channel dimension with the Marketing Channel Details dimension to get an accurate representation of how the Marketing Channels are getting attributed.