在CTS部分看到说插入Boundary Cells
When you are working on a block-level design, you might want to preserve the boundary
conditions of the block’s clock ports (the boundary clock pins). A boundary cell is a fixed
buffer that is inserted immediately after the boundary clock pins to preserve the boundary
conditions of the clock pin.
请问一下什么是boundary conditions,boundary cells在这里的作用是什么呢?