You can only capture skills of your current primary or secondary profession.
After activation, a dialog box will appear that lists all the capturable skills of all dead bosses of your primary or secondary profession within range. Special skills such as monster-only skills will not be listed. You can select a skill and click capture or cancel to instead keep your Signet of Capture.
You can buy multiple Signets of Capture from skill trainers and equip more than one. It is thus possible to temporarily have multiple elites by using multiple Signets of Capture without exiting the area or returning to an outpost. If you have multiple elite skills when you move to a new zone, only the rightmost (highest numbered) elite skill on your skill bar will remain equipped. Remember to rearrange your skill bar before zoning to keep the correct elite skill. The other elite skills will be unavailable until you re-equip, and blank spaces will replace them in your skill bar.
Signet of Capture is a PvE-only skill. Therefore it cannot be equipped by heroes and is subject to the limit of 3 PvE-only skills at a time.
At level 20, capturing an elite skill will yield an impressive 5,000 XP. This is one third of the XP needed to gain another skill point. This will also remove all Death Penalty. Pets do not benefit from this experience.
Signet of Capture cannot be used to capture elite skills from some bosses in Istan. This appears to be related to the levels of the bosses.
In Junundu Mode, the Signet of Capture will remain in the skill bar covering skill in the slot occupied by the Signet. The corresponding Junundu skill is restored once the Signet of Capture has been successfully used. The captured skill is not displayed on the skill bar until the player has left the wurm.
It is impossible to capture elites from another campaign (i.e. through EotN) without having access to that campaign.
While the signet is really only suited for elite skills, consider Prophecies skills where some are only available far in the game from trainers but are available on early/midgame bosses.
Anomaly! Although the skill description states a nearby range, you are able to capture skills from dead bosses who are anywhere within Spirit range.
The concept of the Signet of Capture was first created by the Kingdom of Orr, as found through the quest Capturing the Orrian Tome and the quest Orrian Excavation. Supposedly, the Signet is powered by strong energies found only in bosses. This Signet is then able to permanently transform into a skill of your choice used by the boss affected.