EX: By SDTM definition it says,
1. It captures all the subjects exposure data that is specified in the protocol.
2. It captures the data in units that are specified in the protocol. The protocol will already specify what units are to be used.
EC:
1. As said, subjects might miss their visit or will have unscheduled visits. Then in that case, subject will have different date other than protocol specified date. That exposure data will be collected in EC.
2. Some time local labs will use their own measurement system. If the study is in different countries, then their local lab uses different system units instead of protocol specified units. Those are captured exactly the way they were collected. For example, if the protocol specified units were collected cm and the local lab collected in inches. Then inches are captured in EC.
These are the 2 main difference you may find between EX and EC.