"Calling dibs" - what does it imply?

The term "to have dibs on something" or "to call dibs on something" plays a recurring role in American film and television (e.g. How I Met Your Mother), so it gets exported a lot.

Wikipedia describes "dibs" as

[...] a common, "informal" convention to reserve or declare full or partial ownership of a community resource, such as a chair [...]

From the usages in the media I have seen, this convention also seems to contain some moral concept which is not described in dictionaries. For me, it looks like by calling dibs (or shotgun), the caller actually receives the (implicit, moral) right to that resource, and this typically goes undisputed by their rivals.

Can you confirm the existence of this concept? Is this something that varies between groups of people (or maybe between regions), or is there a concensus on how to respond to this?

I don't see a contradiction (or even much of a difference) between the Wikipedia explanation and your observation.

Though it is a casual practice that wouldn't hold up in a courtroom, calling dibs in the US is common and, within groups of friends/peers, largely accepted. I don't think it's so much an issue of having a "moral right" to something as being a sort of verbal equivalent of "first come, first served."

It's much more common among younger people (and most common among children). The younger the age group, the more binding the "contract."

In American culture, and possibly elsewhere where "dibs" or a word based on the same concept is used, the idea is one of mutual respect for one's claim to some item, position, etc. The adoption of this concept likely originates in childhood interaction, where parents are encouraging respect for other's things, and children, always seeking out fun ways of applying what they are learning, adopt ideas such as "dibs" to express "I would like to claim this, so please respect my right to it."

The general idea is that the person is getting in the first request on whatever it is. There's nothing especially moral about it, other that the "diber" clearly made a request, and if it isn't honored that means the group is effectively denying that request.

Calling "shotgun" is just shorthand for calling dibs on the "shotgun" position in a car (front passenger side).

In Chicago, calling "dibs" on a parking spot is a universally honored, de facto law during the winter season. Street parking in the neighborhoods is always in short supply, and after a snowstorm, the rule is, if you shovel the snow out of your parking spot, for the next few days you are permitted to claim dibs on that spot by placing some godawful piece of furniture (or some other nearly worthless domestic item) when vacating the spot. Here is one website documenting the creativity of some of these dibs claims.