Here's the short summary on how creatures are constructed (The tail goes into body_default):
b_detail_plan: Contains shortcuts for the designation of materials and tissues used in a creature, as well as layering for those tissues.
STANDARD_MATERIALS defines material templates to be attached to the tag (such as HEART or MUSCLE). Material templates are found in material_template_default
STANDARD_TISSUES defines the tissue for that material, also attached to the tag. Tissues are found in tissue_template_default.
TISSUE_LAYERS defines the layering of these tissues for each bodypart, with ARG1-5 defined in creature (usually SKIN, FAT, MUSCLE, BONE and CARTILAGE)
First a bodypart is defined by category (Each bodypart has a category in body_default), then the layers are defined from inward to outward, each with their
relative thickness. For example, [BP_LAYERS:BY_CATEGORY:LEG_LOWER:ARG3:3:ARG1:1] defines LEG_LOWER bodyparts as (in to out) bone:3x, skin:1x (This
being for a modded creature...normal creatures have more layers). ARG can be replaced by the tags mentioned above as appropriate (ex: eye uses the EYE tag)
body_default: Defines bodyparts that will go together to make the creature. They are put together in the creature itself under the BODY tag.
A creature has each of these defined sequentially...for example, an ordinary creature would have:
[BODY:QUADRUPED:TAIL:2EYES:2EARS:NOSE:2LUNGS:HEART:etc...] Chooses the bodyparts to be used.
[BODY_DETAIL_PLAN:STANDARD_MATERIALS] (Taken from b_detail_plan) Defines the materials for that creature.
[BODY_DETAIL_PLAN:STANDARD_TISSUES] (Taken from b_detail_plan) Defines the tissues for that creature.
[BODY_DETAIL_PLAN:VERTEBRATE_TISSUE_LAYERS:SKIN:FAT:MUSCLE:BONE:CARTILAGE] (Taken from b_detail_plan, using the tags defined previously)