When creating Metrics in the Account Control Center, you are able to perform a "Lookup" to another object by using "dot" referencing. What this means, is that if you want to reference a field on an object other than the one you are importing, you can do so by looking up that object through ID fields on the object you are importing.
For example, if you are creating a Metric from Salesforce Product Line Items, and you want to reference the Opportunity that the Line Item is related to, you can reference that Opportunity by using the property
$OpportunityId and then putting a dot, and then adding the Opportunity field name you want to look at.
So, if you wanted to look at the Stage of the Opportunity, then your property would be
And if you want to make a Condition with this metric, then you would put your value after an equals sign. So, if you wanted your Condition to be that the Opportunity Stage is "Prospecting", then your Condition would be
The field before your dot is the lookup field that you want to use to look through to the other object. Sometimes there are multiple lookups to the same object on the same record. For instance, Opportunities will have a CreatedById field as well as an OwnerId field, and possibly other lookups as well, which all lookup to the User table. So, if you want to lookup a property of the Opportunity Owner you would use
$OwnerId. and if you wanted to lookup a property of the person who created the Opportunity, you would use
Also, you are able to look through multiple objects using this same function by stringing multiple dot references together. So, if you are calculating a metric on Salesforce Product Line Items, and wanted to reference the "Start Date" field on the Contract object (and the Contract object is linked to Opportunities), then you would need to do a lookup from the Product Opportunity, to the Opportunity, to the Contract. So your property would be
So, if you want to put a condition that says that the Start Date of the Contract is greater than today, then your condition in this case would be
$OpportunityId.ContractId.StartDate > TODAY()