a paint with good diffusion properties,
can be wiped clean etc
sometimes you hear the expresssion "breathes well" which would be another possibility

what it means in practice is permeable to water vapour

a grab-bag of sample refs:

[PDF] ?Not in my buildingZ<caron>
File Format: PDF/Adobe Acrobat - View as HTML
... easy water vapor diffusion is considered `permeable.' Examples of permeable materials
are concrete block, gypsum wallboard, and certain types of latex paint. ...
www.jm.com/textra/moisture.pdf - Similar pages

Prairie Foam - Icynene Specs
... vapor barrier unless applied to a non-vapor permeable surface in extreme vapor drive
conditions. A vapor retardant paint is adequate in such situations. Water ...
www.prairiefoam.com/specs.htm - 20k - Cached - Similar pages

Chimney Safety Institute of America
... Paint, or any non vapor permeable water sealer, should never be used as a waterproofing
agent because it will trap moisture inside the chimney, accelerating ...
www.csia.org/home/water.html - 14k - Cached - Similar pages

I think diffusionsoffen means the paint will protect against ingress of moisture but will also allow free passage of moisture vapor from the wall.

"The resultant microcrystalline structure has a pore size that allows the free passage of vapour (silicate paints have a very low diffusion resistance equivalent to approximately two centimetres air layer thickness), but the pores or 'holes' are small enough to prevent the ingress of driven rain. The inherent nature of a silicate paint is that of a semi permeable membrane.

I'm making this up, but it certainly seems to fit the context.