Reference: JMX Specification

JMX Model Overrides

The jmx root-level tag defines textures and materials that should be used when JMX is loaded. It will not be used if a frex tag is present and a FREX renderer (i.e. Canvas) is active.

"jmx": {
    "textures": { },
    "materials": { }
}

FREX Model Overrides

The frex root-level tag defines textures and materials that should be used when a FREX renderer (i.e. Canvas) is active. When present and active, it overrides the jmx tag.

A frex block is useful mainly with models that use different textures and material properties when shaders are available. Such models can define a still-enhanced-but-without-shaders appearance in the jmx block that will be used when Canvas isn't available.

This "override" behavior only applies within the same JSON file. Texture and material names within the frex block can reference texture and material names provided via imx blocks in other JSON model files (or even vanilla models for texture names.)

"frex": {
    "textures": { },
    "materials": { }
}