Cautery pen
Cautery pens are handheld medical instruments used to control blood loss. A low temp unit runs at about 1400 degrees. High temp units, most commonly used in branding scarification, are well over 2000 degrees at the tip.
Cautery pens can be used for fine-line scarification, as well as for bleeding control during certain procedures.
Cautery pens have fallen out of favor with many mod artists, for several reasons. They are not powerful enough to create a lasting scar (most cautery pen brands are almost invisible after a year). They are prone to overheating so frequent breaks are needed to allow the instrument to cool down, and they tend to not be that effecting for bleeding control as the tip cools too quickly when it comes into contact with blood. Most practioners these days prefer to use a mains cautery unit, or an electrocautery device.
Soldering Iron
The problem with soldering irons, wood burners, and similar devices (including small handheld medical cautery tools generally used to control small bleeders) is that they don't retain enough heat to be used with any degree of reliability. In order to use them effectively, one must brand with so many "re-heat" pauses that undue pain is given to the client.
If a tool such as these is to be used, it should be as powerful as possible to make the branding experience (and product) a positive one.