Figure 1 illustrates a typical use of a guard condition in a use case diagram. If the customer is using a credit card the Buy Part use case will include the Validate Credit use case. Guard conditions typically get documented as preconditions within the text of the use case.
Figure 1 - Guard Conditions
Use case realizations, sometimes called collaborations, are used to denote a specific implementation of the use case. In the example in Figure 2 the Find Part use case is implemented (realized) by the Parts Locator application. Use case realizations are typically used to show legacy implementations of choice responsibilities in the system. Realizations are denoted by a dashed line with an outline arrowhead.
Figure 2 - Use Case Realizations