A database could have Contacts, Clients, and Vendors - all people, sharing most attributes, but some different.
Clients would appear on Invoices and Payments, Vendors would appear on Purchase Orders.
An argument could be made both ways, I would keep them separate. Use and reporting makes more difference than shared fields.
There's definitely no clear "rules" to tell you what option is best. Some developers just keep to one table and add lots of fields that are empty for all but one type of record. Others use related tables for each, but they often then have a central table for those fields that are common to all categories and use separate related "detail" tables for each type to record the data that's different.