Anyone know why they are called this? Where they meant to be replaced together in groups similar to yarn batch numbers? Or is it marketing fluff?

Group replacement refers to a re-lamping program rather than a specific lamp type. Some fixtures are numerous and in hard to reach places, depending on the building design. Lets take an example of a large department store that has hundreds and hundreds of lamps. Spot-replacement (so only changing EOL lamps) is difficult because the ceiling is high (you need a lift) and it has to be done after hours (for safety). That means sending a crew with the (rented) lift at night to change maybe 10 lamps. Then in a few weeks, they have to do it again. It gets cost prohibitive really fast. With a group re-lamp, they go in after a number of hours, usually close to the rated lamp life (or earlier if the rated lamp life is to 50% failure) and change out every lamp, EOL or not. That way, you only call the lamp maintenance once, and they take care of all lamps and EOL ballasts.

Other examples of group re-lamp that is worthwhile is parking lots, swimming pools, gymnasiums, churches, etc.

Group replacement refers to a re-lamping program rather than a specific lamp type. Some fixtures are numerous and in hard to reach places, depending on the building design. Lets take an example of a large department store that has hundreds and hundreds of lamps. Spot-replacement (so only changing EOL lamps) is difficult because the ceiling is high (you need a lift) and it has to be done after hours (for safety). That means sending a crew with the (rented) lift at night to change maybe 10 lamps. Then in a few weeks, they have to do it again. It gets cost prohibitive really fast. With a group re-lamp, they go in after a number of hours, usually close to the rated lamp life (or earlier if the rated lamp life is to 50% failure) and change out every lamp, EOL or not. That way, you only call the lamp maintenance once, and they take care of all lamps and EOL ballasts.

Other examples of group re-lamp that is worthwhile is parking lots, swimming pools, gymnasiums, churches, etc.

Thanks for the info, but why are the lamps labeled as such?

Is it like replacing all the bulbs and every single fixture. For instance in a gym with 400 watt high bay lights the gym has a few out you replace them all or just at 3?

@Bulbman256 : I would imagine it serves as a reminder to change them all out. It might be a specialty labeling for a specific application.

@ alexd120 : For group replacement in a gym with all MH lamps, you would change them all, regardless of if they are EOL or not. Usually these programs, if done properly, have re-lamping scheduled for when the lamps are getting close to EOL, but with very few failures. Then all get redone at the same time. While not as important for fluorescent, with MH (especially quartz MH), you have the advantage of uniform brightness and almost uniform colour (standard MH lamp colour can vary slightly from lamp to lamp).

My high-school just replaces them as the they go out. There 250w mh and 175w mh can fixtures in the main hall. Each bulb in the main hall way of the school are all a different color

I have a red group replacement Incandescent.

A lot of the incandescent streetlight lamps were group replacement. When I was in school , they used to relamp the entire building every 3 years or so . (F40T12's)

Thanks for the info, but why are the lamps labeled as such?

It means the life rating is specified to be the direct baseline for group replacement schedule (so corresponding to 2% hard failures and output reduction to 70% for streetlights; and not just unusable generic median life rating figure). It could mean the lamp manufacture is optimized for lifetime expressed according to this definition.

Lamps intended for series operation were often marked group replacement, because here it really was very important.  As the filament ages it becomes thinner.  If operated on a constant voltage supply the increased filament resistance causes the power to decrease slightly, which in turn reduces filament temperature and extends the life to achieve its specified value.

If operated on constant current circuits the opposite is true - the thinned filament is driven by the circuit at same current, which causes power to increase and filament temperature to increase, which accelerates end of life.  As such, incandescent lamps run on constant current circuits have shorter life than normal, and the shape of the mortality curve is distorted towards end of life as all lamps rapidly fail faster and around the same time.

Because of this it was far more important to do group replacement on lamps used in such applications, and I suppose they were labelled as such to remind maintenance crews to do that.

The same used to be common on tramcar lamps in Britain, where it was common to use eg 5 lamps in series on a high voltage DC line.  Due to manufacturing variations there are always small differences from batch-to-batch, which causes small voltage differences between individual lamps run in parallel on a constant current circuit.  If one group of lamps had slightly lower voltage than another on the same circuit, they would draw more power and suffer a shorter life.  Sometimes these lamps were marked with voltage bins and those had to be used together on the same circuit.  Since that was not always easy to achieve as not every production batch might have the desired bins, it was customary to replace all lamps together.  Perhaps these types were also marked as Group Replacement in USA.

I knew there was a better reason than what I said. Thank you, James.