A single character. sechar is the maximum size of a character, such that an array of sechars of size X can always hold a string of size X.
secharptrdatum is what an secharptr points to. Such an array does not point to sechar necessarily. For instance, on MBCS, strings and thus secharptr point to an array of characters (bytes). However, each MBCS logical character may take up several of those bytes. Therefore, sizeof(sechar) > sizeof(secharptrdatum) for MBCS builds.