The most tedious and time-consuming phase in a software development cycle is the software testing. White box testing is performed by developers and testing teams dedicated in uncovering defects earlier in a cycle, providing fixes for reported failures, regression testing and debugging in both unit level and integration level testing. Understanding the program units and program boundaries is essential and crucial for measuring a thoroughness and a cost of a testing and efforts related to prioritization of the tests. In this work we try to understand the boundary of a program unit in white box testing of procedural programs and  bjectoriented programs. We extend understanding on boundaries of units under white box testing toward developing understanding on determining a test order that may be effective for prioritizing integration and system level testing.