How emissive it is I would think depends on the values you put in the texture. It's definitely not an on/off thing as far as I can tell from other game engines.
Here's an example of how emmisive textures work in the TMTK:
View attachment 2435
For this till, here is the main texture or 'Base Colour' file:
View attachment 2436
Then here is the emmisive texture:
View attachment 2437
originally, I just had the emissive as large block textures where the screens are, but it totally washes out the design... so a greyscale approach seems to work much better