Tanks in RvREdit
Tanks have multiple supporting tasks, that they should try to do at all times. Their abilities make them necessary for stabilising a group, giving healers a chance to keep the group alive, while at the same time, they can assist in killing their group's foes.
Tanks in RvR are the most effective characters to protect others by using abilities, that they all share.
Using guard on players, that are attacked, splitting the damage between tank and guarded player, is considered the basic job of any tank, no matter what spec this specific tank has.
The range of guard is small, so staying more or less on top of the guarded player is necessary to prevent lag, unlucky movement or cc from removing the effect by having the guardee out of range.
For times, when guard is insufficient or the tank can't get into range, there are three different damage reductions avaible.
For one, there is the ability "Challenge", that reduces the damage others take from challenged players by 30%, while you would still take full damage.
Second there is a shared morale 3 ability called "Distracting Bellow", that reduces the damage all affected enemies do by 50%.
As a last resort, ther is the shared morale 4 ability "Immaculate Defence", that reduces the damage you and your group takes by 75%.
Assisting others means contributing to the damage a single target takes, by attacking, debuffing it or buffing your group.
Since DPS-classes provide higher burst, the tanks job is not providing raw numbers, but making sure, that there is enough damage around to keep the enemy's healers busy.
That can be archieved by different means and in different ways by the classes, but most of the time it boils down to hitting the same target your guardee or assist-lead, one of the most important positions in a group, does attack.
For buffing and/or debuffing all tanks have specific abilities. Most of them are bound to attacks, so choosing those to attack does help your group more then choosing something, that does a minor bit more damage.
Most DPS prefer having a higher chance to critically hit(for example initiative-debuffs or direct buffs to critchances) and lower defences(armor debuff, weapon skill buffs, toughness debuffs) and some buffs to their damage(strength/intelligence/ballistics buff).
The strongest debuffs avaible are always healdebuffs, since the difference between healing done and damage recieved is small even without them.
Swordmaster specifics in RvREdit
Swordmasters are a tank-class, that is better at assisting then at protecting their allies. Therefore most choose to increase their damage output, making them offensive supporters.
There are only two swordmaster specific abilities worth mentioning here, since most of their abilities provide no defensive benefit to their group.
"Dragon's Talon" is a low duration damage debuff, that can be used on a single target. Hitting the right target can limit the enemies burst by an amount large enough to make their target healable.
On the other hand, the PBAE-Kick provides a second chance for kiters to escape. Due to low range and long imunities, this ability is best used rarely, after all other options are depleted.
While assisting in RvR, swordmasters usually use "Nature's Blade" to buff their team with stats they randomly steal from enemies around them.
Using either "Aether Dance" or "Dazzling Strike" they provide a meaningful addition to burst, making it more likely to kill a target before enemy healers can react.
Depending on what other tanks the group has, swordmasters can also take care of silencing healers, preventing them from healing, or knocking down tanks to prevent them from being in guard-range. Both of these options require speccing for the related cc.