In the context of 3D modeling and material properties, transmission and opacity are both associated with adjusting the degree of transparency or opacity of a material. However, they work conversely with each other.
Transmission: Transmission is the degree to which a material allows light to pass through it. Ranging from 0-1, the higher the transmission value is, the more translucent the material becomes. At its lowest transmission value (0), the material is completely opaque, and at its highest (1), the material is translucent with a slight tint of its base color.
Opacity: Contrary to transmission, opacity is the degree to which a material blocks light attempting to pass through it. Ranging from 0-1, the higher the opacity value is, the more opaque the material becomes. At its lowest opacity value (0), the material is completely clear, without any tinge, and at its highest (1), the material is totally opaque.
It is important to note that only either opacity or transmission need to be adjusted, not necessarily both at the same time. For example, regardless of the transmission value assigned, the opacity value will still take full effect on the material, and vice versa.