Restrictions on values permitted for compartment size and units attributes.

Value of spatialDimensions	size allowed?	units allowed?	Allowable kinds of units	Default value of attribute units
"3"	yes	yes	units of volume, or dimensionless	"volume"
"2"	yes	yes	units of area, or dimensionless	"area"
"1"	yes	yes	units of length, or dimensionless	"length"
"0"	no	no	(no units allowed)