allbd - Returns the names of all loaded block diagrams

Names of all loaded (or partially loaded) models and libraries are returned in a cell array

This very simple function is just a shorthand for the find_system call which returns the names of all loaded block diagrams- models and libraries. Note that loaded block diagrams are not necessarily visible, especially if they are libraries.
Note also that the find_system call is not necessarily what you might expect to use: it is the fastest- but perhaps not the most intuitive- way to find the list of loaded block diagrams. The command we might think of first would be:
>> names = find_system('Type','block_diagram')
while the command used here is:
>> names = find_system('SearchDepth',0)
By specifying a "SearchDepth" of zero, we ensure that Simulink doesn't search through the entire object hierarchy looking for objects whose "Type" parameter is equal to "block_diagram". And since we know that all objects at SearchDepth zero are block diagrams, we don't even need to specify a type.