Channel XML: overall systematics shared by different samples

Roofit and RooStats
1

Hi everybody,
I have a question about the use of histfactory to create workspaces combining several samples within the same XML channel file.

Suppose, for example, that I have this setup for the XML channel file (it’s just an example):

<!DOCTYPE Channel  SYSTEM 'HistFactorySchema.dtd'>

  <Channel Name="channel1" InputFile="./data/example.root" HistoName="" >
    <Data HistoName="data" HistoPath="" />

    <Sample Name="signal" HistoPath="" HistoName="signal">
      <OverallSys Name="syst" High="1.05" Low="0.95"/>
      <NormFactor Name="SigXsecOverSM" Val="1" Low="0." High="3." Const="True" />
    </Sample>

    <Sample Name="background1" HistoPath="" NormalizeByTheory="True" HistoName="background1">
      <OverallSys Name="syst" Low="0.95" High="1.05"/>
    </Sample>

    <Sample Name="background2" HistoPath="" NormalizeByTheory="True" HistoName="background2">
      <OverallSys Name="syst" Low="0.95" High="1.05"/>
    </Sample>
  </Channel>

So I include signal and background samples with the same overall systematic “syst”.

I would like to know how histfactory treat this systematic while creating the workspace.
The 3 samples are obviously different so the bin in each histogram are differently populated;
so, if I consider the sum of the 3 samples I should obtain from the OverallSys a contribution similiar to a shape systematic (each bin of the sum has a different scale factor).
Said that, I wonder how the histfactory combine the same systematic across different samples, and if I can consider that systematic ad an overall systematic even after the combination.

Thank you very much.