Here's how you can get the approval limits for PO hierarchy:
1. Identify the position (attached to an employee) for which you want to find out the approval limit.
2. Then go to Purchasing > Setup > Approvals > Approval Assignments. Search by the above position.
3. In the search results, note down the 'Approval Group'.
4. Now navigate to Purchasing > Setup > Approvals > Approval Groups, and search using above approval group. There you will be able to see the approval limits. This approval limit will be associated with the above position (which in turn maps to the employee to which the position has been attached).
My requirement got multiple conditions for setting up the approval rule i.e. combination of COA and item category with priority; so first of all, I need to understand how the system to retrieve approval limit, if it falls into multiple rules, also where can I customize the logic if the standard does not support the requirement.
Its best you refer to the Purchasing User guide and read the documentation. I wouldn't recommend customization and suggest you to try using the standard functionality as far as possible.
Here's the resource mentioned above:
Oracle® Purchasing User's Guide
Part No. E48931-01
Link to access this user guide = Oracle E-Business Suite (12.2) Documentation Library
If you have multiple conditions for approval rule then you need to use Oracle AME - Purchase Order Approval.
It provides lot of flexibility in terms of determing approval limit based on conditions which can be dynamic based on a SQL or can be static as well. There are plenty of material in the net on using AME for PO Approval.
As Neerej suggest, try to utilize AME for your condition. Its rule based approval engine and you can have more flexibility and control the approvals based on multiple criteria.