fetching related records from Leads and Cases, which is present in Contacts module

fetching related records from Leads and Cases, which is present in Contacts module

Hi,

We have a custom field 'Reference Number', auto increment number in Contacts Module. We are able to fetch the record using getSearchRecords API. 

https://crm.zoho.com/crm/private/xml/Contacts/getSearchRecords?authtoken=...&scope=crmapi&selectColumns=All&searchCondition=(Reference Number|=|EZ1020)&newFormat=2


But we are not able to find a common field which links Contacts and Leads & Contacts & Cases.

Which is the common field and how / where to determine or set this up.

regards
Mani