Declare a column of data

Decide whether a column of your data is a dimension, a relation or a parameter. What decides is what the math does with the column.

The column…	is declared as	because
is an axis: something is indexed by it, or an aggregation lands terms on it	a dimension	its members are the coordinate set every table over it is reindexed onto
has one value per member of a dimension, or per tuple of several — a generator's bus, a line's two ends, a generator's zone by period	a relation with that key	it is a map every operator reads, and its values are checked against the dimensions they name
relates members of two dimensions many-to-many, with nothing to weigh — which buses a generator may connect to	a bare relation, with no values:	sum reads it with both ends named, and a bare where tests it. Nothing reads it, because there is no one value to read
relates members of two dimensions many-to-many, with a weight per pair — a link's efficiency to each bus, a cycle's lines	a parameter over both	the weight is the data, its row set is the relation, and the aggregation is sum(w * x, over=a)
is a label set the model only selects on or counts within — a period, a season, a zone	a dimension, and a relation onto it	its labels are checked, at the cost of one line and one table
scales terms — a coefficient, a bound, an offset	a parameter (float or int)	arithmetic is over numbers (dtype)
is a per-row attribute the math only selects on — a fuel, a constraint's sense	a str parameter	it names rows rather than scaling them, and no set is declared to check its values against
is a mask	a bool parameter	a bare name in a where is its own answer

Two rules decide the cases the table does not list:

1. If b has one value per a, declare b as a relation keyed by a.
2. Two dimensions that depend on each other are one relation.