Use the Buyers window to define and maintain your buyers. Buyers can review all requisitions using the Requisitions window, and only buyers can enter and autocreate purchasing documents. See the Document Types window for rules governing access to documents
• Define employees before performing this step. See the online help for the Enter Person window for details.
• Define locations before performing this step
HRMS Management -> HRMS Manager -> People -> Enter & maintain
Purchasing -> Setup -> Personnel -> Employee (Only if HRMS is not available)
Enter the Name of an employee who you want to define as a buyer. If the name you want does not appear in the list of values, use the Enter Person window to enter that person as an employee
Position Hierarchy : http://www.oracleug.com/user-guide/hrms/position-hierarchy