Separating solids - how to determine which body stays on same layer

[d.spreemann]

When separating solid bodies (Solidedit > Body > Separate) with the solid on one layer & my current layer is different - how do I know which body stays on the solids layer & which change to the current layer?
It seems to be rather random - sometimes the selected portion stays on the solid layer & sometimes the selected portion changes to the current layer
Am I missing soemthing obvious?

[pendean]

DWG file please.

[d.spreemann]

The attached drawing (this was done with AS but the same thing applies with all AutoCAD flavours). 

1 - 2 solids intersecting eachother - 1 on trash layer & other on standard layer. Trash layer is current

2 - Subtract trash solid from standard solid leaving standard solid in 2 parts but still 1 piece (not sure that came out properly)

3 - copy 2 to 3 & Solid Editing > Separate > select top part = top piece stays on standard layer & bottom piece moves to current layer (trash)
4 copy 2  to 4  & Solid Editing > Separate > select bottom part = top piece stays on standard layer & bottom piece moves to current layer (trash)

So is there any way to predetermine which part(s) move to current layer & which part(s) stay on its layer?