The top level tags [multiplayer], [test], [tutorial] and [scenario] are all formatted the same way. (A description of scenario formatting can be found in ScenarioWML). The difference between these tags is the way that the scenarios they describe are accessed.
There are two ways to play a scenario. One way is to win a scenario of the same tag type with the value of next_scenario equal to this scenario's ID. If you do this, your units from the previous scenario will be available for recall. The other way to play a scenario is if it is the initial scenario of a campaign.
A list of initial scenarios, and how to access them:
All multiplayer scenarios are initial scenarios listed in the multiplayer scenario selector screen (accessed by the "multiplayer" button).
The test scenario with the attribute id=test is an initial scenario. This test scenario can be accessed by running the game in test mode. (note: this is NOT the same as debug mode. It can be accessed using -t or --test)
The tutorial scenario with the attribute id=tutorial is an initial scenario. The tutorial is accessed by clicking on the "tutorial" button.
Any scenario scenario with an id listed in the value of first_scenario in a campaign tag (see CampaignWML) is an initial scenario accessed by selecting that campaign after clicking on the "campaign" button.