Nothing, but both should be expressed as undimensionable coeficients; i.e. refering to unit pitch (1 module or 1 DP) and in the same plane. An unqualified (transverse) add mod coef on a helical will always refer to the normal plane.
Maag Gear Book by Maag Gear-Wheel Company, Zurich Switz'
(paraphrases Swiss Std VSM 15525)
Gears by H E Merrit, pub' by Pitman
German standard DIN 3960 clause 2.5.4 and beyond.
Dudley (US) & Tuplin (UK) do not handle this subject well.
In my experience, US engineers I have worked with have been at best perplexed by the concept at worst frightened to death by it. There are limits as how far one can go defined by undercutting (-ve) & tip loading & pointing (+ve). Outside a fairly tight range in which slide roll ratios can be improved on non-unity ratios, capacity will be compromised. I've had to resort to case hardening where 250BHN sufficed on non-corrected gears. DIN 3960 fig no. 11 gives guidelines for practical limits. It does (& has) offer good solutions to many gear design problems.