Pierro groupcap

Can anyone explain to me what the pierro groupcap actually is?

Espresso parts says this:

This kit removes the external plumbing components from a Linea AV model group, to a GB5 AV Style group, reducing the amount of time the water travels before being dispensed through the grounds, thereby retaining it’s temperature.

pretty pricey though.

A Linea classic’s water path takes water from the grouphead and runs it through tubes outside the head, through the solenoid and flowmeter, back into the grouphead through a tube that dispenses water on top of the dispersion screen. The Piero cap takes the solenoid and flowmeter and mounts them to the top of the group head, so water doesn’t ever leave the head. It helps with temp. stability.

i know its standard on the gb5, but does the linea pb have this also?

Yes, the PB has them, and they can be retrofitted to the Classic too.