SI units. All SI units and their abbreviated forms are included whether attested or not.
ISO language codes are included under the ==Symbol== header, again whether attested in the regular fashion or not.
Words from languages without the necessary literary base. These do not require regular attestation, and inclusion is determined by the consensus of the community.
Numbers are included only if the entry supplies further information than could otherwise be figured out from other entries, regardless of whether the terms are attested. Thus billion, ninety and eighteen should be included, because further information can be given about it. However, three hundred and twelve should not be included. An exemption is made to the rule for all integers under one hundred.