Proportionality and Server expression for a series

Dear Friends,

I have a requirement to create a series and show forecast manually uploaded and incase it is null, i should show the system generated forecast.

I was going through series definitions and explanation and expressions to get this requirement done.

I am confused with server expression and proportional in series option and no where it is written the exact need for server expression.

If we keep a series 'Y' proportional to series 'X', then value of series Y at higher level(Product Category) will get proportionally split to lower level(SKU) based on proportion of splitting of lower level at series 'X'.