Clarification require regarding schema tables/custom schema registration

We have the two tables FND_APPLICATION and FND_ORACLE_USERID, We have below questions around these tables:

1. what is the relevance of FND_APPLICATION and FND_ORACLE_USERID

2. is there any relation between FND_APPLICATION.APPLICATION_ID and FND_ORACLE_USERID.ORACLE_ID

3. we are currently at EBS 12.1.3, in process of upgrade to 12.2.4, we found that the custom schemas are not available at FND_ORACLE_USERID table at 12.1.3, however after registration the custom schemas populated at this table, currently the custom schemas are not registered in application. 

if we will register the custom schemas at 12.1.3 only and get them populated at FND_ORACLE_USERID, will be get any issue in the application?