Are you using Allow Virtuals and Use Billboards? There’s a function for Ralent Tracking that tries to determine the position of the talent in 3D space by optically looking at where the talent’s feet ends. It’s not a perfect solution, since there can’t be one. The video is just a 2D image after all, so there’s no real spatial awareness of where is is in the room. There’s also an option to put a tracker on the talent, which gives more reliable results if you set it up correctly. 

when I turn it on (Allow Virtuals and Use Billboards) the input turns upside down and becomes a billboard does this work right?

that is, there is no way other than to attach the MARS tracker to a person and somehow track his position?

Like I said, you can also use the optical talent tracking, which works without a tracker. No idea why it turns upside down, but try disabling any rotation on the Billboard position when enabling Billboards and auto tracking. 

"Optical talent tracking  "     tell me please  where i can find that option  ? 

No need to answer, i find the setting , its in the  Inputs of tracked cam -> Billboard 1A -> Auto position dropdown menu 

Hi,

If only the smoke or fire stayed behind the billboard and Use Billboards plus Allow Virtuals fixed it, then the problem was probably caused by additive materials in the particle system. More on it here: https://my.aximmetry.com/post/1515-virtual-camera-issue-202141---urgent
If you are good with Allow Virtuals, then you don't have to worry about this problem anymore.

Warmest regards,