Set of nodes and chains. Any transaction tree for a given specific address is contained entirely within the cell. In the case of a transaction between cells, there are two transactions, one inside the cell, the other outside. Nodes are linked by links both inside the cell and outside. Full nodes can be limited to links within cells, while master nodes can create external links.
In the code, a cell is just an ID that is declared by the cell declaration. This ID is contained in the Data element (Datum) and related with the nodes and chain which should be segmented into parts.
Important: if the Node is configured as non-celled, it contains all cells of the network.