The supply and waste lines which pierce the slab are put in place before the slab is poured - in fact, before it is even set up to be poured. The most important thing to check here is the location of all pipes which the plumber has installed. You can do this by measuring from the foundation walls (if these are used) or the forming for the edge of the slab.
Precise positioning is important so that pipes to be located in walls actually end up in the walls, and toilets are properly located.

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