How to create OTBI Custom Subject Area

Summary:

While creating OTBI custom Subject Area, how to know which primary object is for the specific table?

Content (please ensure you mask any confidential information):

The background is that we have the requirement to convert the Sales Order ordered amount into local currency and USD currency,

but we found below

(1) The sales order transactional currency column is NOT existing in Order Management related Subject Areas.

(2) There is no GL daily rates related Subject Area.

So we wants to know how to convert the Ordered Amount to other currencies?

While testing to create OTBI custom Subject Area, how do I know which primary object should I choose (For example, which Primary Object is for GL_DAILY_RATES table) ?