It is characterized by the omission of general clipping operation thus the user can define graphics elements ( point line circle ) interactively and precisely acknowledge them to effect intersection and tangent and trace them to form a 2-D loop data structure .

本法的一个显著特点是省略了通常的裁剪过程，在图形元素定义、几何交切和跟踪成环过程中自动 裁剪，并 可重叠跟踪。

FISUs carry no user data but can be used to maintain link integrity and acknowledge MSUs .

FISU不传送任何 用户数据，而是用于维护链路的完整性和 确认MSU的。