Dimensions, Transactions and custom attributes in BAM

Hi experts,

I’m creating KPIs for my BPM flow, not able to understand the importance of Dimensions and Transaction attributes and where do we see them (in monitoring)? Becuase, I didn’t find them used in either the customised KPI or the default KPI.

I also need an info on , whether there is a way to add custom fields in the KPI?

Please provide me your inputs.