A block type defines the common properties for a category of blocks.
Block types are defined in block type diagrams, and these may be referenced by means of block type references.
Block types may contain a connectivity graph of block instances connected by channels. This makes up a structure of nested blocks. At the leaves of this structure there are blocks which contain processes. In SDL, block types may not contain both blocks and processes at the same time.
In addition to containing structures of blocks or structures of processes, block types may contain other type definitions. This makes up the scoping hierarchy of SDL. Names in enclosing type definitions are the only names visible.
Block types may contain data type definitions, but no variable declarations. This follows from the fact that processes in SDL do not share data other than signal queues. They share a signal queue in the way that one process appends (output) signals to the queue (the input port), while the other process consumes (input) signals from the same queue. Appending and consuming signals are atomic, non-interruptible operations. The input port is the basic synchronisation mechanism of SDL.
Block types may contain process types, service types and procedures as well as block types and data types.