It is when the organism does things to "cultivate" it for some purpose. Like pheremones for example, or the fragrance of flowers which attracts pollinating insects. Or even the horrible stink like rotten corpses from flowers that rely on flies for pollination.
Dont female dogs give off a 'smell' when in heat...I can remember when I was a kid, the lady dogs in the neighborhood, when they went into heat, drove our male dogs nuts...our dogs would lay at the back door, and groan and groan, because they wanted out, and wanted to get at that lady dog....
I remember when we lived next door to a minister, when I had my own kids, and he used to lock his female dog up in a huge outdoor area, completely enclosed by fencing...you could look outdoors anytime of the night or day, and see a pack of male dogs, hanging around, desperate to get into the fenced in area...the minister even had to put fencing on the bottom of the area to prevent the male dogs from digging underneath the fence...
No one told these male dogs that girlie dogs were in heat....they could 'smell' it...