What does B line mean on the reservoir? And is possible/bad to overfill with too much coolant?

When I do a fluid change, I fill it to the B line. Twice I have not had to add anymore
fluid.

 

Yes, the B line is explained in the repair manual in the instructions for replacing coolant; it's the line you fill to before burping the air out.

Toyota Service Information and Where To Find It | PriusChat

In a nutshell, you fill to the B line, put the cap on, run the engine up to operating temp and yea minutes more (squeezing and shaking some hoses doesn't hurt either, just watch your fingers around the fans), shut the engine off, and wait for it to completely cool down. Once the coolant is totally cooled to its normal room-temperature volume, look at the level again, it should be right around F. (Every time I've done it, it landed right on F at the end, just like the engineers knew what they were doing.)

 

Baseline.

 

B for burp? That's funny! (Probably true, too!)

No, you can't overfill coolant BUT it's a mess when too much comes out of the overflow tubes so it's best to stick with the lines on the reservoirs. (I keep many flashlights handy to help see them...especially the hidden brake fluid reservoir!)

 

Whether it stands for Burp or Bleed or some other English b-word that a Japanese translator might have thought of for getting the air burped out, we might not strictly know, but any of those would give you the right idea, as you see in the repair manual instructions for what you use it for and when.

Guessing 'baseline' would lose you your turn, because that isn't how it's used.