TIDM codes are used to identify UK listed securities. They were previously called EPIC codes.
TIDMs are short and are usually based on an abbreviation of the name of the company. For example, the TIDM for Glaxosmithkline ordinary shares is GSK and the TIDM for Vodafone ordinary shares is VOD. They are memorable enough for people to remember, and are commonly used to enter orders into trading systems and to call up quotes and other information related to a security.
The disadvantage of TIDM codes is that they are a UK only system. Although equivalents exist in most markets, there is no co-ordination or standardisation. Therefore systems that must cope with global trading generally use ISIN codes.
There are some codes that combine TIDM codes (and their equivalents elsewhere) with a country identifier in order to combine the memorability of TIDM codes with guaranteed global uniqueness (like ISIN codes). The most important of these are Reuters and Bloomberg codes. Both add a country code suffix. The Reuters code for Glaxosmithkline is GSK.L and the Bloomberg code GSK LN.