What parameter allows a view to be joined multiple times?

The correct answer is the "from" parameter. In LookML, the "from" parameter is used in join definitions to specify the name of the view that is being joined to the primary view. This is essential when you want to join a view multiple times, as it allows you to define different instances of the same view under different aliases. By doing this, you can create multiple joins to the same underlying data structure, each utilized for different filtering or analytical purposes.

The ability to reference the same view multiple times is crucial for scenarios where you need to compare data across different dimensions or aggregate data in various ways without duplicating the view's definition. This flexibility is pivotal in crafting complex data models where relationships between data points have to be analyzed distinctly.

In the context of the other parameters, while "join_type" specifies how the join operation behaves and "view_key" typically pertains to the uniqueness of a view, it is the "from" parameter that explicitly allows for multiple instances of a view to be joined within a single explore. The "relation" parameter is not a standard LookML parameter, which further clarifies that the "from" parameter is indeed the correct choice for this query.