A Data Element is the logical definition of a unit of data such as:
a column in a database table
a field in a form
an element in an XML schema
Date of birth
Sex
Blood type
Data elements can be found contained within data classes. For example, the data class Participant might include the data elements: first name, surname, participant ID, date of birth, sex, etc.
Data Element values are constrained by Data Types. For example, the data element Date of birth may be associated with the data type xs:date, which only allows for the input of data in the YYYY-MM-DD format.