He's talking about an additional light that replaces the tailcap on the standard (or the rechargeable) MAG. It comes in red, amber, or blue, and it has a strobe beacon inside the colored lens/dome.
I've often wondered the same thing. Here's what little I've found. They have LOTS of them at Gall's "returned-item" sale every month. I asked why, and they told me the most frequent reason wasn't dissatisfaction, but ordering the tailcap light for the wrong flashlight. (The tailcap strobe is available for the traditional Mag (C-cell or D-cell), Mag-Charger, and Streamlight SL-20X and SL-35X. This offers many opportunities for confusion when ordering.)
Me, I figure I'll pass on this item, and save my battery life for the main light.
Here's a
link to a picture and description, along with some reviews, the gist of which is, "Neat product, but way too pricey."