DB2 Reorg-recommended commands
A table is placed in the reorg pending mode if you alter its physical structure, such as add or drop a column or change the column data type or nullability. 

Some operations do not place the table in reorg pending, but reorganization is required for the changes to take effect, for example, changing the table COMPRESS attribute or the inline length of a LOB column.

Adding or dropping a foreign key does not place the table in reorg pending, nor does it require reorganization. However, changing an existing foreign key from not enforced to enforced will put the table in the check pending mode, which, like reorg pending, prevents applications from accessing that table.

I don't know of a single good source to read more about reorg, apart from the DB2 manual, particularly the ALTER TABLE statement and this article:

DB2 10.5 for Linux, UNIX, and Windows: Altering tables