Size comparison of transistor packages. Two surface-mount packages, SOT23 and SOT223, are shown next to through-hole packages
A small outline transistor (SOT) is a family of small footprint, discrete surface mounttransistor commonly used in consumer electronics. The most common SOT are SOT23 variations,[1] also manufacturers offer the nearly identical thin small outline transistor (TSOT) package, where lower height is important.[2][3][4][5]
The SOT89-3 electrically only has three leads (contact/pin). The wide lead (tab) is physically part of the middle lead on the other side of the package. Some call this package a SOT89-4, since it visually appears to have four leads when looking down at the part.
The SOT89-5 electrically only has five leads (contact/pin). The middle lead is physically part of the middle lead on the other side of the package. Some call this package a SOT89-6, since it visually appears to have six leads when looking down at the part.
^Hollander, Dave (1995). "Packaging Trends and Mounting Techniques for Power Surface Mount Components". Proceedings of 1995 International Conference on Power Electronics and Drive Systems. PEDS 95. 1: 264–270. doi:10.1109/PEDS.1995.404911. ISBN0-7803-2423-4.
Note: It's relatively common to find packages that contain other components than their designated ones, such as diodes or voltage regulators in transistor packages, etc.