Correct. Whatever test kit you are using will dictate how many slots you will have to use. Reefbot only allows certain brands of test kits for the different tests and the bot will allocate the correct number of vials/locations when you setting up any particular test.
I have had mine set up for a few weeks now and I also run the Trident tester. I have found pretty large discrepancies between the two units and I'm sure they may have some to do with different kits/reagents. For example, my Trident will say my calcium is at 440 and at the exact same time the Reefbot will say it's at 510. Initially I used my Hanna tester and found my calcium in this example to be 460. Therefore, I'm not 100% certain what to believe but I have decided that the Trident and Hanna to be closer in readings so I stick to that thought and focus on keeping the numbers stable regardless of which tests tells me whatever.
Since I have the Trident tester I mainly purchased the Reefbot to help me keep tabs on my nitrates and phosphates but since I had open slots I decided to have a "second opinion" on the calcium and alkalinity. For the record, the alkalinity is also measurably different than the Trident. The bot gives me 10 consistently and the Trident goes from 8.2 to 8.5 daily. I have the Trident set up for 4 tests a day. I used API test reagents in my bot mainly because I have a substantial amount of them, as I use API for my freshwater tanks so when I initially started with salt tank I just gravitated to that brand. After my API depletes I will be trying other brands that the bot allows just to see if I get similar readings.
Would I buy the bot again if I knew what I know now? Yes, not so much for the 3 top tests, as I feel more comfortable with the Trident for those, but definitely for the nitrates & phos. Even if they are not spot on reading wise, for me they are close enough and reassure me that my parameters are at least stabile/consistent.
Ok....so that was a LOT more information than you asked for...LOL