Intuition behind valuation of Portfolio and Composite? guess, why, individual, valuation, different

laurab

1.Why is “time weighted using beginning of period asset values and external cash flows” mandatory for portfolios
while
2. Asset weighting individual portfolio eturns OR a method that reflects both beginning of period assset values and external cash flows allowed for composite?
I’m too tired and unimaginative now to guess why composite treated different from portfolio