The Switch Cancel is executed by pressing the tag buttons in the middle of a combo, allowing a player to switch characters (with the exact timing for each combo varying). The tag partner will be swapped in as the current character is executing their move. Executing a Switch Cancel uses one section of the Cross Gauge.
Switch canceling is one of the most vital mechanics of Street Fighter X Tekken. Any hit or block can be switch canceled allowing potentially more damage than the use of Launchers, however it is advised to only do it with attacks that have allow for followups or safety.
Tag Canceling resets the juggle point counter (except on Cross Cancel) allowing for extended combos. This can help in increasing damage in combos. Be warned that damage scaling does not reset. This mechanic also allows grapplers (like Zangief) come in off a crumple switch cancel to hit their high damage Command Grab. Arguably the best time to switch cancel is off an attack that allows a combo off a grounded opponent (such as Ibuki's Hien), as most of the cast can do more damage off ground hits or rehit with a 100 damage launcher so the starting character can cause even more damage.
While Tag Canceling has great offensive usage it is also good defensively when used properly. Raw switching has very little safety so finding great opportunities to Tag Cancel is paramount to success. The 2 main ways to find safe Tag Cancels are off attacks that have high amount of blockstun (such as Rufus's Galactic Tornado) or have a long range attack that the incoming character can CADC away.
While Tag Cancels are great, their usage of the Cross Gauge puts them in direct competition with EX attacks, the user must take care to balance the usage between both mechanics for optimal results. In general; Tag Cancels do more damage but sacrifice any Provisional Damage.