How a hammock task is defined depends on the how the task is used in the parent project.
Definition 1The Project Management Institute defines a hammock as "An aggregate or summary activity. All related activities are tied as one activity and reported at the summary level". (Definition taken from the PMBOK - Project Management Body Of Knowledge.) Based on this definition, you can use Microsoft Project to create a hammock task by using the outlining feature and sunmary tasks.
Definition 2A second definition for a hammock task is a task that will occur between two dates, neither of which necessarily correspond to the task's start or finish. In Microsoft Project, this type of task is generally depicted as a normal task having dependencies with a milestone representing the early date for the task start and a milestone representing the latest date that the task may conclude. The task may have dependencies other than those defined with the milestones.
Definition 3Another definition for a hammock is a task that derives its start and finish dates through links pasted from other tasks in the project. For information about how to create this type of hammock task, please see the following article in the Microsoft Knowledge Base:
141733 How to Build a Hammock Task