MultitypeTree migration rate interpretation

[geo]

Dear all,

I have some doubts in the interpretation of the values of migration rate in MultitypeTree .

What is the difference betwee forward and backwart?

I'm modelling the migration of a virus among different host categories.

In particular, I see that they are not symmetric. Therefore, I don't know how to report the migration rates among different  categories. 

Thank you so much for any help you could provide me

Giovanni