I've often seen it mentioned in the money/gifts for sex or services realm. For instance, some (especially professional) dominants and sex workers request or demand a money/gift "tribute" from those who are interested in receiving services, and it's often framed as 'send me XYZ to show me you're real and truly serious about being with me.'
So, if you see it mentioned in ads for erotic services or whatever, it very likely means someone is requesting or offering something in exchange for attention or to show genuine interest of some sort.