GF: The maximum duration of the secondment depends on the length of the fellowship – what is the defining rule here, the months oft he outgoing phase (so 3 months secondment), or the overall length of the fellowship, or the two years of the return phase?

The secondments must be clearly justified in the Part B of the proposal and their maximum duration is defined according to the total duration of the fellowship:

 Therefore, as you rightly assumed, for a Global fellowship of 3 years, the maximum duration of secondment is 6 months.