DOMAIN (V023) defines the basic geographic units for which the sample was designed to yield representative estimates.
The DHS Recode Manual describes DOMAIN as follows:
For example, if the sample was designed to be self-weighting within region, this variable would define those regions; if the sample was designed to be self-weighting within major urban areas, other urban areas and rural areas, this variable would define the major urban, other urban and rural areas. If the sample is self-weighted at the national level, this variable is code 0.
When national estimates are desired, users should include survey design variables, including DOMAIN (V023) and PSU (V021), and the correct sample weights (e.g., PERWEIGHT (V005)) to adjust sampling errors to account for DHS' sample design.
In most countries, DOMAIN defines the stratification for the sample (generally either regional or national). However, some countries use two levels of stratification, such as region and urban/rural areas. Users should check for the correct stratification design by consulting the Final Report(s) for the sample(s) of interest (particularly the Introduction and Sample Design sections and Implementation Appendix).