How the IsVisible works is described there Restricting Visibility of a Plugin to Specific Entries in CM.
See below examples as well as clip properties that can be accessed with the get operator.
|get('duration') > 8000|
Plugin is visible only if the duration of the roughcut is higher than 8sec
|get('mediumType') == 'Graphic'||Plugin is visible only if the medium type of the clip is a Graphic (picture)|
Only for clips. You can access clip properties as below by using the get operator.
|get('duration')||Returns the duration of a clip|
|get('offset')||Returns the offset of a clip|
|get('title')||Returns the clip title|
|get('mediumType')||Returns the medium type of a clip. Returned values are 'Audio' or 'Video', or 'Graphic'.|