Question

Does the RER only synthesize protein to be transported outside the cell or those within the cell or both?

Answer 1

Each cell's RER produces proteins to be used by the cell itself and to be secreted
.

Explanation:

Proteins to be secreted follow the secretory pathway: rough ER → Golgi → secretory vesicles → cell exterior.

Proteins who are to be used by the cell follow the same pathway but without the secretory vesicle and the cell exterior steps.

Answer 2

In Short
Proteins synthesized by RER is utilised by cell itself , it is not transported outside the cell ( it may be used as the cell membrane protein etc)

While the proteins synthesized by Ribosomes are transported from one place to another according to their demand of protein.