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.

Examples

IsVisibleDescription
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)

Clip Properties

Only for clips. You can access clip properties as below by using the get operator.

GetDescription
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'.