Difference between Substitutes in Private Off settings & Workflow settings

All,

I see two Substitutions in SAP Business workplace. In SBWP, Can anybody explain the difference between the substitutions in SAP Business workplace we can set up a Substitute thru Settings ---> Office Settings ---> Substitute.

Another Substitute is in Settings->Workflow Settings>Maintain Substitute.

I read through the help and understood their functionality. But, not so clear. So, can somebody explain the difference between these two(if any) and in which context what is used.

Pl. reply asap.

Thanks in advance

venu