  In the paper we discuss certain classes of vector distributions in the
tangent bundles to manifolds, obtained by series of applications of the
so-called generalized Cartan prolongations (gCp). The classical Cartan
prolongations deal with rank-2 distributions and are responsible for the
appearance of the Goursat distributions. Similarly, the so-called special
multi-flags are generated in the result of successive applications of gCp's.
Singularities of such distributions turn out to be very rich, although without
functional moduli of the local classification. The paper focuses on special
2-flags, obtained by sequences of gCp's applied to rank-3 distributions. A
stratification of germs of special 2-flags of all lengths into singularity
classes is constructed. This stratification provides invariant geometric
significance to the vast family of local polynomial pseudo-normal forms for
special 2-flags introduced earlier in [Mormul P., Banach Center Publ., Vol. 65,
Polish Acad. Sci., Warsaw, 2004, 157-178]. This is the main contribution of the
present paper. The singularity classes endow those multi-parameter normal
forms, which were obtained just as a by-product of sequences of gCp's, with a
geometrical meaning.
