The rules that use the category "<altitude>" are:

<air_location>

<bearing_range> <air_speed> <altitude>

<bearing_range> <air_speed> <altitude> <heading_aspect>

<bearing_range> <altitude>

<bearing_range> <altitude> <air_speed>

<bearing_range> <altitude> <air_speed> <heading_aspect>

<bearing_range> <altitude> <heading_aspect>

<bearing_range> <altitude> <heading_aspect> <air_speed>

<altitude_directive>

<altitude_direction_verb> <altitude>

<altitude_direction_verb> <altitude> <direction_prep> <point_name>

<vector_spec>

<heading> <altitude>