Doubt in Activity No assigned to Package Number

Hii

ME53N - T-Code..Under a Purchase Req. No there are many Services...A single service no/Activity(SRVPOS) may have more Pkg No(PACKNO)...

how can i find a Service belongs to which Unique Pkg no??

How this Package no is assigned to Activity/Service no??

Thanks in Advance

Satyabrata

ABAP Consultant