What the BTA204-600C,127 actually is
The WeEn BTA204-600C,127 is a standard triac in a TO-220-3 through-hole package, rated for 4 A RMS continuous on-state current and 600 V off-state blocking voltage. It is a three-quadrant device — meaning it can be triggered in quadrants I, II, and III, but not IV — which is the typical behaviour for most AC phase-control and on/off switching applications. The 35 mA maximum gate trigger current and 1.5 V maximum gate trigger voltage tell you the drive side: a standard optocoupler or a small-signal transistor can fire it, but you need to budget for that gate current at the MCU I/O or the driver transistor's collector. The 20 mA holding current means the load must draw at least 20 mA to keep the triac latched after the gate pulse ends; below that it drops out at the next zero-cross, which is fine for most resistive and inductive loads but worth checking for very light loads like a small fan or a relay coil.
Where it fits on the board
The TO-220-3 package (supplier device package TO-220AB) is a standard through-hole power package. The tab is the main terminal (MT2), so the heatsink — if you need one — is electrically live. Plan for an insulating pad and a mica washer if the heatsink is grounded. The part is rated for a junction temperature of 125 °C, so thermal management is on you: for a 4 A continuous load at 25 °C ambient, a modest clip-on heatsink is usually enough; derate if the ambient is higher or if you are switching near the surge limit. The bulk packaging means it ships in a tube or tray, not tape-and-reel — fine for through-hole assembly but not pick-and-place.