The general principle is that the mobility allowance is spent on costs related to the ESR’s mobility (e.g. relating to travel and accommodation). This allowance covers private costs of the researchers.
The Research, Training and Networking costs are spent on travels in connection with the project (professional costs).
Additionally, the EC has suggested that possible visa costs can be taken from the RTN costs as well.
You can find some guidance in the AMGA, p. 455 and 461.