The name is from JEDEC, signifying Transistor Outline Package, Case Style 18.
For example, diode lasers such as those found in CD players may be packaged in TO-18 cases with a lens.
[4] The fourth wire is typically connected to the metal case as a means of electromagnetic shielding for radio frequency applications.
TO-206 is intended to replace previous definitions of packages with leads arranged in a circle with a diameter of 2.54 mm (0.100 in).
A new package with 3 leads and a cap height of 1.40 mm (0.055 in) (i.e. smaller than TO-46) is added as TO-206-AD.