Question about boundary command

Dear All,
I want to simulate a system, imposing PBC only in two dimensions.
I read the guide, in particular the BOUNDARY COMMAND, but I can not understand the meaning of styles non-periodic and shrink-wrapped and non-periodic and shrink-wrapped with a minimum value
Could you help me?

shrink-wrapped simply means a non-periodic system where the
box-size continuously adjusts to the extent of the atoms in that
dimension. Thus is one atom floats away from the free surface,

it won’t be lost (as it would with “fixed” BC), but the box will

grow as needed to include it.

If you start with an empty box (e.g. for pouring granular particles
into), then shrink-wrap would give you a zero-size box, so the minimum
value simply means it won’t shrink below some minimum value.