A CAD document is made up of data, metadata. Some of this comes from CATIA such as the geometry, mass and material. Some of it comes from the PLM, Design code and RPG. 
Two pieces of metadata can be changed in both the PLM and CATIA, the CAD definition and description. These two properties are mapped in both directions so can be modified in either platform and the data will be updated in the other. However, its important to understand when this information is transferred, so as not to loose any data.

• Data is pushed from the PLM to CATIA when a document is first opened.
• Data is pushed from CATIA to the PLM when a document is saved.

Why this is important? 

If you have a document open in CATIA and change either the CAD definition or description in the PLM platform, these updates will not be pushed to the already open document in CATIA. The CATIA data will not be updated. Therefore, if the CAD document is subsequently saved in CATIA, the 'outdated' CATIA data will be pushed to the PLM overwriting the previous change. 
Therefore, please do not change a documents metadata in the PLM if the document is already open in CATIA. Please do the changes in CATIA.

Note: If a document is modified in the PLM whilst it is open in CAD, when the CATIA tree is refreshed the definition will be updated because this displays data directly from the PLM connector. However, the CATIA data is not updated. This can be verified by viewing the data via right click > properties.