You can't speculatively call dibs, for example if somebody brings out tray of cakes you can call dibs on the largest one, but you cannot call dibs on the biggest cake when there is no cake in question, just to have an ongoing dibs on all future cake situations.

The general term of dibs is overruled by more specific equivalents where relevant. For example you can call dibs on riding shotgun in a car, but if somebody actually calls shotgun that overrides the dibs. (Dibs cannot be overruled by preference though, for example if you dibs the passenger seat, and nobody outranks you with a call of shotgun, the driver cannot say "Actually I wanted my girlfriend next to me" she just has to step her game up for the next journey!)

Dibs only covers material things, for action based calls bagsy outranks dibs. For example you would call dibs on the last slice of pizza, you would call bagsy for next turn playing a video game. (Bagsy can be reversed, for example when the activity is a chore you can call 'bagsy not me' Dibs however can only be used positively, so you can't 'dibs' the smallest slice of pizza to someone else)

If two or more people synchronise their dibs callings to the point that there was no discernible first place, a best two out of three rock/paper/scissors will decide the victor.