Paper: Enhancing SfePy with Isogeometric Analysis (Robert Cimrman)

Hi! Here it is...
NURBS requires some more explanation, given how central it is to IGA and the examples. Somewhat later the abbreviation is defined as "Non-uniform rational B-splines", but that's not enough for a non-expert in the field.

Figure 1 shows "a domain with NURBS boundary". I suppose the boundary is the outermost line shown. So what are the lines inside? Are they part of NURBS? Do they define a decomposition of the domain that is used in IGA? What are the advantages and limitations of describing domains by NURBS boundaries?