A variable's dimension variances specify whether or not the values change along the corresponding dimension. The effects of a dimension variance are defined as follows:
The values do change along the dimension. All of the values for the dimension (or all of the subarrays) are physically stored.
The values do not change along the dimension. Only one value (or subarray) is physically written for that dimension. Each value (or subarray) along that dimension is the same (including virtual values/subarrays beyond the first value/subarray).
Figure 2.2 illustrates the effect of dimension variances on a variable with 2-dimensional arrays (for a particular record). For variable 1 each value in the array is physically stored and therefore unique. Because variable 2 does not vary along the second dimension, each value along that dimension is the same so only one value for that dimension is physically stored (the other values are virtual). The same is true for variable 3 which does not vary along the first dimension. Variable 4 does not vary along either dimension. Only one value is physically stored for the array --- all of the other values are the same (they are virtual).
Figure 2.2: Physical vs. Virtual Dimensions
A variable's dimension variances are specified when the variable is created. The dimension variances of an existing variable may be changed only if values have not yet been written to that variable. (An explicit pad value may have been specified, however.)