According to the American Heritage Dictionary, a "flanker" in markting terminology is "An extension product, such as a diet version of a soft drink or a liquid version of a detergent, added to a line to support the sales of the main product."
In perfumery, it designates a perfume which was issued as an extension of an original perfume. There are different types of fragrance flankers. Here are some critical definitions to, hopefully, clear up the confusion.
An Olfactorily Relevant Flanker vs. a Fake or Creative Flanker
A fragrance flanker can be olfactorily relevant, i.e., retain the main characteristics that make up the original composition, or it can be more symbolic - a reinforcement of the brand name of a pillar perfume.
With an OR Flanker, you nose recognizes the original scent despite olfactive variations, with a Creative Flanker you could get confused because the names are similar while the scent is completely different...
The Dior Poison franchise is an example of a series of "fake flankers". They have kept the name of the original Poison and the shape of the bottle, but the compositions of Pure Poison, Poison Tendre, Hypnotic Poison and Midnight Poison are all distinct, standalone ones.
"OR flankers" are to be found for instance in the exponentially developing series of Narciso Rodriguez perfumes which are very consistent at promoting their olfactive signature through variations within their main collections of Narciso Rodriguez for Her and Essence.
A Lineage Flanker vs. a Faithful One
There are also flankers which share a similar name and borrow something from the original pillar perfume while being more interested in reworking and renewing the olfactory signature of the perfume composition as much as can be while keeping a lineage olfactive quote inserted in the formula. It is a way of paying homage to tradition while making sure that the more general olfactive signature of the house gets imprinted on our psyches.
At Guerlain, Shalimar Parfum Initial is a Lineage Flanker of Shalimar while Shalimar Parfum Initial L'Eau is a Faithful flanker to its predecessor cited above.
Flankers which Stay within One Olfactive Family
Once a perfume and then a perfume franchise become popular, it becomes hard to resist the temptation of having more of the same; a style is born. Recreate and remix the same ingredients such as the names, the spirit of the collection, the main olfactive orientations, the expectations. Keep people guessing - but not too much - as to what the next issue will be. Flankers then become both familiar and new sharing many similar traits.
Escada is a good example of where a series of flankers is expected to go each and every year in the continuum of sweet, lactonic fruity-florals. If they put out a dry chypre all of a sudden, it would feel like someone drank out from one of their bottles and did not get the marketing brief straight.
While some perfume brands prefer to keep it quiet or call it a new creation with recurring themes, the fact is that some new perfumes feel like they are twists on previous ones. But this is not advertized. Oftentimes a perfume creator hesitates between different directions. Alternate formulas are literally kept in drawers to be whipped out later on whenever the time feels ripe for the development path which was set aside.
Serge Lutens is a good example of a creative brand which offers both OR Flankers, e.g., L'Eau and L'Eau Froide, and Artistic Flankers, e.g., Féminité du Bois and its descendants, which are unadvertized as such. One could argue also that many of his perfumes offer a variational quality which yet retain an air of creative purpose which prevent them from coming across as overly formulaic.