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.