If I don't remember wrong, jBM5 will get all the information in the content field and store that as a byte array to send the information to the human task server. If you don't add anything into the content field automatically jBPM will copy all the input mappings into the content in order to transfer it to the human task component.
As Mauricio said the content parameter is used to pass all info for a specific human task, to the human task server. To achieve this you have two main methods,
1. Set data to the Content parameter from the field, as you have done. This way you may only pass String data
2. Set data to the Content parameter from a parameter mapping, to pass any type of data. Check out the following link,
To access the data you may do it inside the ftl as shown in the link above, or via code/API check the following links,