What is the meaning of accumulation

The word "accumulation" is used in many of the definitions in the sense that th e horizon must have more of the material
in question than is presumed to have been present in the parent material. A Bt horizon is a B horizon characterised by clay accumulation (t); likewise in a Bk horizon carbonates (k) have accumulated. When more than  one suffix is used, the following letters, if used are written first: a, e, i, h, r, s, t and w.