Yes, the mastering tools are limited to EQ, compression & normalisation so no reverb. You can apply reverb to any/all tracks during the mixdown process, so these FX will appear in the master file which is created at that point. You could also apply reverb by bouncing track(s) to another track. These would then contain reverb before you start to mixdown.
When you press Play in mixdown mode, you can rehearse the mix and check FX and fader movements etc. All the faders will function. When you're happy, press Record and adjust faders as rehearsed and you'll create a master file which should be exactly as you just heard it. If you mess up, press Record again and have another go; the file will be overwritten.
Don't forget in order to playback this master file just created, you need to go into Master mode by pressing the Mixdown/Mastering button again - the faders will no longer have any affect. If you stay in mixdown mode and press Play, you'll just hear the individual tracks again via the faders.
Tascam confuse things a bit (imo!) by using the term 'master file' for both processes. There is only one file but its contents are either the result of mixing down or mastering, so it might have been better to just call it a 'composite' file. Either way, it's completely separate from the individual track files. Hope that helps.