The termination of the first-ascent red giant luminosity function near helium ignition provides a calibrated population-II distance indicator. In suitable passbands the tip luminosity varies relatively weakly over a useful old, metal-poor population range. Colour/composition corrections, foreground extinction, photometric completeness and contamination by other evolved stars must be controlled; an arbitrary brightest red star is not a reliable tip measurement.
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The "tip of the red-giant branch" (TRGB) refers to a specific stage in the evolutionary life cycle of stars, particularly for stars with masses similar to or slightly greater than that of our Sun. A star reaches this phase after it has evolved off the main sequence, having exhausted the hydrogen in its core and transitioned to hydrogen shell burning outside of an inert helium core.