Here is a list of all documented struct and union fields with links to the struct/union documentation for each field:

arch : ORDO_VERSION
build : ORDO_VERSION
drop : RC4_PARAMS
feature_list : ORDO_VERSION
features : ORDO_VERSION
id : ORDO_VERSION
out_len : SKEIN256_PARAMS
padding : CBC_PARAMS , ECB_PARAMS
reserved : SKEIN256_PARAMS
rounds : AES_PARAMS
schema : SKEIN256_PARAMS
system : ORDO_VERSION
tweak : THREEFISH256_PARAMS
unused : SKEIN256_PARAMS
version : ORDO_VERSION , SKEIN256_PARAMS