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In general, selectively permeable membranes allow the passage of only certain and highly specific molecules or substances through them.
It is the same in the case of plasma membrane.
Plasma membrane allows the movement of only certain substances like hydrophobic substances through it.
It doesn't allow larger molecules and even hydrophillic or  water soluble substances to pass through it.(but sometimes even these hydrophillic  substances enter the cell cytoplasm with the help of transport or carrier proteins.)
Plasma membrane is highly specific.
Hence, it is called as a selectively permeable membrane.

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Plasma membrane is called selectively permeabe membrane because only some specific materials are allowed to pass through it.