Custom Entities
Microsoft creates over 150 entities when the Microsoft Dynamics CRM software is installed; however that does not mean you can not add more without donating an arm and leg to a team of developers. In fact you can add an almost unlimited number of custom entities and the possibilities of what to do with them is a very, very, very long list; however there are some limitations that you might want to be aware of.
1) Merging custom entity records together can not be done.
2) You can not create a custom entity that has a relationship to a existing system composed entity (such as the select from either/or Account or contact in a single lookup)
3) When looking at an Entity rollup, Custom Entities do not appear.
4) Parental behavior with system entities cannot be done with custom entities