Sem contexto é dificil. Para mim trata do sistema de trading da London Stock Exchange. Vê links abaixo

http://www.bes.pt/sitebes/cms.aspx?plg=15001B21-C591-4924-82...
http://www.londonstockexchange.com/en-gb/products/membership...

Mas sem contexto é dificil mesmo vês estes outros links :
http://www.crest-approved.org/
http://www.crestmgi.co.uk/

Espero que estou a ajudar !

CREST is an electronic system for settling the sale and purchase of shares. Membership is voluntary and is typically used by institutions, and by individual investors who have a large share portfolio or who buy and sell shares frequently.

CREST enables shareholders to hold and transfer their shareholdings in electronic form rather than using paper.

For further information on CREST please write to CRESTCo Limited, Trinity Tower, 9 Thomas More Street, London E1 9YN.


E também:

Campbell R. Harvey's Hypertextual Finance Glossary, onde se lê:

CREST is CrestCo's real-time settlement system for UK and Irish shares and other corporate securities. From 1999, CrestCo will also take ownership of government bond and money markets settlement in the UK.