What is anticollision, and when does it matter?
Anticollision is the procedure a reader uses to single out one tag when several respond at the same time. It works by narrowing down identifiers until only one remains. It is a solved problem at the protocol level, and it does not rescue tags that are physically interfering with each other.
The problem it solves
A reader has no way to address a specific tag before it knows one is there. It sends a general call, and every tag in the field answers at once.
Those answers overlap and are unintelligible. Without a way out, a reader faced with more than one tag would be permanently stuck, which is unacceptable in a wallet, a stack of cards or a shelf of tagged items.
How the narrowing works
The procedure uses the identifiers. The reader asks tags whose identifier begins in a certain way to answer, listens, detects whether the answers still collide, and refines the question.
Each round eliminates candidates until exactly one tag remains, at which point the reader selects it and everything that follows is a private conversation. The other tags stay silent until they are addressed in turn or the field goes away.
This is why identifiers exist in the first place. Distinguishing tags during selection is the original job, and every other use of the identifier is a repurposing.
Why it is not your problem
The procedure is defined in the standards, implemented in every compliant reader, and does not need configuring. As a buyer of tags or a builder of a deployment, there is nothing here to specify, tune or worry about.
What it does mean in practice is that a phone presented with several tags will pick one, deterministically enough at the protocol level, and arbitrarily from your point of view. Which one you get is not something you control.
What it does not fix
This is the part worth remembering, because it explains a failure people misattribute constantly.
Anticollision is a protocol solution to a protocol problem. Two tags stacked against each other fail for a physical reason instead: each absorbs field the other needs, and each detunes the other's antenna. Neither tag receives enough energy to answer properly, so there is nothing for the procedure to arbitrate between.
The symptom is identical to having no tag at all, which is why "the reader should just pick one" feels like it ought to work. It cannot, because the collision the standard handles is a collision of answers, not a collision of antennas.
The practical rule: separate tags physically. Anticollision handles tags that are near each other, not tags that are touching.
Frequently asked questions
How many tags can a reader handle at once?
The procedure scales to many, and dedicated readers routinely single out tags from a substantial group. Phones generally select one and get on with it rather than presenting the set, which is a software choice rather than a protocol limit.
Can I choose which tag the phone selects?
Not through any ordinary interface. If you need a specific tag, the reliable method is to present it alone, since the selection is not something applications are given control over.
Why does my card stop working when it is in a wallet with others?
Usually physical interference rather than anticollision. Contactless cards packed together absorb and detune each other, which prevents any of them from being read properly.
Does anticollision slow down a read?
Marginally, when several tags are present, since it takes extra rounds to narrow down. With a single tag the procedure completes almost immediately and is invisible.