The skeleton starts with a ValveBiped.Bip01_Pelvis bone, it will be the root bone of all the model's skeleton.

This is why addons such as JMod or WiltOS's animation base won't work properly with playermodels using custom skeletons instead of the ValveBiped one. Using the ValveBiped skeleton instead of a custom one will help developers to design addons, otherwise they would have to think about every custom skeleton on the workshop. The Valve standard skeleton is named ValveBiped and all of its bones must have specific names to be used as a ValveBiped model. In the end, if you ever want to use the default animation pack, you will have to rig your model to this ValveBiped skeleton and include the said model in your QC file using this QC command. This set of animations is packed in a model called m_anm.mdl (and other gesture files), it has been made for a special biped skeleton vastly used in Source Games and its source files are available at this GitHub repo. Most of the time when making a brand new playermodel, you will want to use Garry's Mod's default animations as making custom animations can be very time-consuming or you might just lack the skill for doing it.

Pmc models gmod file names