If I create a booking and only select profile instances for the different functions, the bookings wizard is already proposing resources. How does DataMiner SRM select these resources, and can I be sure that these resource can execute what is actually required for the booking?
DataMiner SRM has a concept of capacities and capabilities to manage resources correctly. Based on the booking profile information that the user enters, DataMiner will check that information against the capacities and capabilities of a resource, and therefore always pick the resources that can provide what you're asking for.
When defining a profile parameter into the profile manager, that parameter can be set as a category 'Capability'. That indicates that that the value that is entered while creating a booking for this parameter, should be also checked against the resource capabilities. These capabilities can be provisioned onto the resource, indicating exactly what the resource is capable of doing.
So, when a booking is created and a profile instance is selected which does contain a profile parameter of category 'Capability', it will automatically filter out the resources which offer that specific capability. By result, DataMiner can always provide the correct resources.
In a similar way, a profile parameter can also be set as a category 'Capacity', and capacities can be defined on resources. When e.g. a profile instance with a specific bitrate is selected, DataMiner SRM will check if that specific capacity is still available, of course for the timestamp that you have defined into your booking. That also ensures that the necessary capacity is available.
Note also that you can define multiple capacity and capability parameters on a resource, it is not limited to just 1. So, you can always be sure the correct resources are picked for your booking.