Mira Mode Instructions Manual page 13

Unvented Mains Pressure System - The shower can be installed with a unvented,
stored hot water cylinder. Only "a competent person" as defined by the Building
Regulations may fit this type of system.
Mains Pressurised Instantaneous Hot Water System - The shower installed with
systems of this type is supplied hot water via a tempering valve, this provides
relatively constant hot water and the shower will compensate for temperature
changes should they occur.
13