This element represents a resource that is used to process the entities. Each resource has a number of available units (capacity). Entities can seize a variable number of resource units before being processed.
Althouhg this model is an element, it behaves more like a
block.
It receives seize petitions (through port S) and depending on the
available number of resource units, these petitions are inmediately
served or has to wait for more idle units. When a seize peition
captures the required units, the resource element sends a
confirmation through the O port to the seize block that sent the
petition.
It also receives release petitions (through port R) that free resources and increase the number of available units.