TL is FL180
TA is 18000
Transition Altitude. The altitude at or below which the vertical position of an aircraft is controlled by reference to altitudes.
Transition Level. The lowest flight level available for use above the transition altitude.
Transition Layer. The airspace between the transition altitude and the transition level.
(ICAO, e.g. Doc 4444: PANS-ATM & Doc 8168: PANS-OPS).
In the US and Canada, the transition altitude is fixed at 18000 feet and the airspace above is known as the Standard Pressure Region
Under conditions of QNH at or above 1013 mb, FL180 becomes the lowest useable FL. If the pressure is lower, the lowest useable FL becomes FL190 or even FL200. This restriction ensures that a minimum of 1000' vertical separation is maintained between the aircraft at 17000' on QNH and the one at the lowest useable level on Standard Pressure Setting.
Guidance on changing barometric pressure setting:
Descending, even when cleared to an altitude at the time cruising level is vacated, the altimeters will remain on standard pressure until just prior to the transition level.
Keep in mind that, in reality, the transition level "floats" based on the atmospheric pressure at see level (QNH) - the lower the QNH, the higher the transition level - If the QNH is low and the pilots wait until approaching FL180 to change the altimeter subscale, an Level Bust is possible.
Both primary altimeters are changed at the same time. In most modern aircraft, not doing so will result in some form of an altimeter missmatch error. Depending upon company SOP, the standby altimeter might also be changed at the transition altitude/level or at top of climb/top of descent.
Source: https://skybrary.aero/
The reason is that half of the US is covered by mountains to >14,000', so an accurate local Altimeter setting is far more important than using an area setting. Some fun trivia too - minimum IFR terrain clearance over mountainous areas is 2,000', so FL180 still provides minimum clearance if the Altimeter swings from 31.00 all the way to 29.00 (1 inHg = 1,000') above the Rockies. Which would be a pretty incredible swing from high to low.