In a human task you need to assign task's to users. In a multiple user store, there are multiple user domains. With the multiple user store support in WSO2 carbon platform (WSO2's middleware platform), WSO2 Business Process Server allows to assign human tasks to users in different domains.
Eg: Lets assume you have ‘ClerkUser’ in ‘abc.com’ domain and ‘ManagerUser’ in primary domain.
So If you are assigning any task to these users they should be referred as follows,
ClerkUser : abc.com/ClerkUser
ManagerUser : ManagerUser
If you want to try this out, setup a new user store as explained in Working with User Stores. Then deploy the ClaimsApprovalTaskWithMultipleDomains.zip human task package in the <BPS_HOME>/repository/samples/humantasks folder inside the WSO2 BPS 3.5 release. Read the sample section of the documentation for more info.