A block is a meta-data representation of a control block.   It has an external interface and an optinal internal interface. Blocks internals can be ommitted if the block is well understood by both exchange parties.    When well understood by both partice the block can be treated as a primitive block.   All dynamic models must be defined to the level of primtive blocks in order for the model to be consumed and used for dynamic simulation. Examples of primitive blocks include a well known IEEE exciter model, a summation block, or an integrator block.