According to the portal article on field modifications, "all tier VI–X vehicles with access to Field Modification receive a special parameter: role; there is a total of 15 roles". I couldn't find a list of roles and bonuses anywhere, so i made one for tier 10s using the tank browser tool in that article.
Versatile LT: +3% hull traverse, +5% hit points
Wheeled LT: +5% traverse
Support MT: +3% engine power, +2% camo
Versatile MT: +5% engine power
Sniper MT: +3% engine power, -3% hull dispersion
Assault MT: +3% engine power, +2% hit points
Support HT: +3% hit points, -3% hull dispersion
Breakthrough HT: +3% hit points, +2% engine power
Assault HT: +5% hit points
Versatile HT: +3% hit points, -2% aiming time
Sniper TD: -3% dispersion on hull traverse, +2% camo
Assault TD: -3% dispersion on hull traverse, +2% hit points
Support TD: -6% dispersion on hull traverse
Versatile TD: -3% dispersion on hull traverse, +3% hull traverse speed
SPG: -3% dispersion on hull traverse, +3% hull traverse speed
You get these bonuses increased gradually as you raise your field modification level (with exp or free exp or a combination of both). The numbers listed above are for the max level on tier 10s. Exp requirements for max levels for all tiers are:
- tier 6: 17500
- tier 7: 35000
- tier 8: 69000
- tier 9: 140000
- tier X: 224000
Consider disabling crew acceleration temporarily on the tanks you want to enable field modifications, so you don't have to waste free exp.
Bonuses for lower tiers are smaller, but not necessarily the same (i noticed tier 9 SPGs have -3% dispersion on hull traverse and +1% accuracy).
There's not much material on this subject – how to prepare for the update or what modification builds are worth looking into, so any constructive discussions or guide/video links are more than welcome.