  The X-ray Integral Field Unit (X-IFU) is the cryogenic imaging spectrometer
on board the future X-ray observatory \textsl{Athena}. With a hexagonal array
of 3840 AC-biased Transition Edge Sensors (TES), it will provide narrow-field
observations (5$^{\prime}$ equivalent diameter) with unprecedented high
spectral resolution (2.5 eV up to 7 keV) over the 0.2 - 12 keV bandpass.
Throughout its observations, the X-IFU will face various sources of X-ray
background. Specifically, the so-called Non-X-ray Background (NXB) caused by
the interaction of high-energy cosmic rays with the instrument, may lead to a
degradation of its sensitivity in the observation of faint extended sources
(e.g. galaxy clusters outskirts). To limit this effect, a cryogenic
anti-coincidence detector (CryoAC) will be placed below the detector plane to
lower the NXB level down to the required level of $5 \times 10^{-3}$
cts/s/cm$^{2}$/keV over 2 - 10 keV. In this contribution, we investigate ways
to accurately monitor the NXB and ensure the highest reproducibility in-flight.
Using the limiting science case of the background-dominated observation of
galaxy clusters outskirts, we demonstrate that a reproducibility of 2\% on the
absolute knowledge of the background is required to perform driving science
objectives, such as measuring abundances and turbulence in the outskirts.
Monitoring of the NXB in-flight through closed observations, the detector's
CryoAC or the companion instrument (Wide Field Imager) will be used to meet
this requirement.
