For each recognizable element there is a function of form
struct TPF_NS_EEE_s* zx_DEC_NS_EEE(struct zx_dec_ctx* c);
where TPF is the prefix, NS is the namespace prefix, and EEE is the element name. For example:
struct zx_se_Envelope_s* zx_DEC_se_Envelope(struct zx_ctx* c);
These functions work much the same way as the root decoder. You should consult dec-templ.c for the skeleton of the decoder. Generally you should not be calling element specific decoders: they exist so that zx_DEC_root() can call them. They have somewhat nonintuitive requirements, for example the opening <, the namespace prefix, and the element name must have already been scanned from the input stream by the time you call element specific decoder.