是要设置参与者和用例间通信的消息的箭头，我看了参考手册，如下：
If you want to add an arrow to indicate the flow of information, do the following:
1. Double-click the Communicates shape, then, under Association, click the end you want to edit, and then click Properties.
2. In the Association End category, check IsNavigable, click OK, then click OK again.
3. Right-click the Communicates shape and click Shape Display Options. Under End options, select End navigability, and then click OK.
----------------------------------------------
我按照他的做了，但是我双击线出来后的对话框中没有【IsNavigable】这一项啊，我该怎么做？
------解决方案--------------------------------------------------------
我用的是VEA2003版本，不知道你用的是哪个版本，我这个版本不是你所说的那样，