How to save non-model artifacts from a container (output_data_dir)

**What did you find confusing? Please describe.**
I am using docker images with my custom training algorithm. I am interested in sending information back from the container (for example, saving coefficients from the fitted model on an S3 bucket). I am confused about the use of the folder opt/ml/output. I understand that files on the folder opt/ml/model will be saved to the output bucket, but what happens to files on opt/ml/output? Can they be saved as well?

**Describe how documentation can be improved**
There is very little information about the output_data_dir. This directory is defined on the environment.py module on the source code, but it seems unused on the other modules. How does sagemaker save files from the output folder to S3?
There is also very little information about the use of opt/ml/output besides the file failure.