Warrant card

A warrant card is proof of identification and authority carried by police officers and some other law enforcement officers including immigration officers. The term is normally used only within the United Kingdom and in current and former Commonwealth countries. Many other countries refer to their equivalent of warrant cards simply as police credentials, commission books, or identification cards. In the UK MI5 officers also have warrant cards. Specially trained mental health practitioners called Approved Mental Health Professional are also warranted council officers and carry warrant cards when acting under the Mental Health Act 1983.
Warrant cards generally include a photograph of the holder as well as the holder's name, rank, warrant number and a holographic emblem to mark authenticity. The warrant number is equivalent to a badge number in other police services; it is a unique identifier unlike a collar number (sometimes displayed on the uniform) which may change when transferring between departments or changing rank. A warrant card is sometimes displayed alongside a badge showing the service to which the officer belongs, but is increasingly commonly displayed on one ID card, with the force crest printed on it.
The text on a warrant card usually indicates that the holder is granted authority by a specific official to perform the functions of the office held, and may also indicate training to a particular level. In the UK, police personnel authorized to carry firearms may have an endorsement on their warrant card to that effect.
United Kingdom.
In the United Kingdom, a warrant card is evidence of a constable's sworn attestation. The power of a constable is in the person and not the warrant card (with the exception of a few services not sworn in by the Police Act 1996 or the Police (Scotland) Act 1967). A constable still holds the power to make an arrest off-duty and without a warrant card. However, force policy usually dictates police officers in plain clothes are required to identify themselves and produce their warrant card when they are performing their police duties and exercising their police powers, so long as it is practicable to do so (for example, not necessarily if the person they are arresting is being violent). Generally, police officers are required to produce their warrant card when requested, even in uniform, but only if it is practicable.
Special Constabulary.
Special constables are sworn officers as a Crown servant and have the same powers as regular officers. As such, they also carry a warrant card. "Special Constable" may be printed on the card along with, or instead of, "Police Officer".
Service/military police.
Service police officers of all ranks also carry a warrant card as issued by their individual service.
Canada.
Toronto.
Toronto Police officers are issued with warrant cards contained within a wallet. When opened the wallet has a metal badge of the force with the officer's identification number, rank and name.
Hong Kong.
Customs and Excise Department.
C&E issued three types of warrant cards: two are for customs officers and one is for trade control personnel.
Smart warrant cards are written in both Chinese and English. The card is yellow in colour for customs officer and grey for trades control personnel. The front of the card contains the name and rank (and UI for customs officer), along with the photo of the individual. The card is covered with a hologram made up of the logo of C&E.
There are three different versions of the obverse:
Police force.
Smart Warrant Card have on the front: "Hong Kong Police" (in Chinese and English); the force arms in colour; the name, rank and UI of the warrant card holder; and a photograph. The card is covered with laser etched "Hong Kong Police" in English and Chinese. On the reverse, the cards bear the statement: "The person whose photograph and particulars appear on the reverse is a duly appointed member of the Hong Kong Police". Allegedly, a number of Hong Kong Police failed to display their warrant cards during the series of anti-Extradition protests. Police later clarified that they were not required to do so, under certain circumstances.
Singapore.
In Singapore, police officers also carry their warrant card. The warrant card allows a search to be carried out by the police officer, however, only the Volunteer Special Constabulary is not allowed to utilize it when they are off-duty. 
The warrant card has security features designed to prevent identity fraud through impersonation as police officers. When the card is tilted at an angle, the holographic word “POLICE” appears below the officer’s photograph, while on the right of the card, a vertical row of five holographic police crests is visible.