For a new field there are 2 options that interest me: one is list selection and multiple selection list, the latter as the name suggests is to select two or more options simultaneously, but it did not work, only works like selection list, why?

For example for a contact I create a field called "campaign of interest" and here I put several options X, Y, Z, A, B, C, then there are contacts that applied to XY, others for A, B, Z others, etc.

But when selecting any option only allows a choice if I want to add another previous clears, why this happens?