Here's how it works:

1. Synthesis: Ribosomes on the RER synthesize the digestive enzymes, which are proteins.

2. Folding and Modification: As the proteins are synthesized, they enter the lumen of the RER, where they undergo folding and modification. This includes glycosylation (adding sugar chains) and other modifications important for proper enzyme function.

3. Transport: The enzymes are then packaged into transport vesicles that bud off from the RER and move to the Golgi apparatus.

4. Further Processing: The Golgi further modifies and sorts the enzymes.

5. Packaging: The enzymes are finally packaged into secretory vesicles.

6. Exocytosis: These vesicles migrate to the cell membrane and release the enzymes into the extracellular space (like the lumen of the stomach or small intestine).

Therefore, the ER is the primary intracellular source of digestive enzymes, playing a crucial role in their synthesis, modification, and packaging.