material_instances definition in Resource or Behaviour Pack?

[elfin sleet]

"Now that texture shortnames have been defined, the game still needs to know what textures to use for the blocks. Blocks will use the material_instances component to tell the game which shortname to use. Add the following component to the appropriate blocks." - https://wiki.bedrock.dev/blocks/blocks-intro.html
Where to define the material_instances? In the block files in the behaviour pack or somewhere else in the resource pack?

From @torn condor

[torn condor]

Thanks

[elfin sleet]

@torn condor To answer your question, material isntances is defined im the BP while the shortnames are defined in the RP