  Characterising the properties of exoplanet atmospheres relies on several
interconnected parameters, which makes it difficult to determine them
independently. Planetary mass plays a role in determining the scale height of
atmospheres, similarly to the contribution from the average molecular weight of
the gas. We investigate the relevance of planetary mass knowledge in spectral
retrievals, identifying cases where mass measurements are needed for clear or
cloudy and primary or secondary atmospheres, along with the relevant precision,
in the context of the ESA M4 Ariel Mission. We used TauREx to simulate the
Ariel transmission spectra of representative targets of the Ariel mission
reference sample, assuming different scenarios: a primordial cloudy atmosphere
of a hot Jupiter and a hot Neptune, as well as the secondary atmosphere of a
super-Earth that also exhibits a cloud presence. We extracted information on
the various properties of the atmospheres for the cases of unknown mass or mass
with different uncertainties. We also tested how the signal-to-noise ratio
impacts atmospheric retrieval for different wavelength ranges. We accurately
retrieved the primordial atmospheric composition independently from mass
uncertainties for clear atmospheres, while we found that the uncertainties
increased for high altitude clouds. We highlight the importance of the
signal-to-noise ratio in the Rayleigh scattering region of the spectrum. For
the secondary atmosphere cases, a mass uncertainty no larger than 50% is
sufficient to retrieve the atmospheric parameters, even in the presence of
clouds. Our analysis suggests that even in the worst-case scenario, a 50% mass
precision level is enough for producing reliable retrievals, while an
atmospheric retrieval without any knowledge of a planetary mass could lead to
biases in cloudy primary atmospheres as well as in secondary atmospheres.
