Maybe I misunderstood what you meant by a "pull elbow". What I was referring to when I said "jake" is something like this:

A unilet is an LB, LL, LR, T or C (straight through)

Jakes and LBs would most often be used around outside corners. LLs & LRs the same but they just locate the cover differently for access requirements.

If by "pull elbow" you meant this:

Then the uses for elbows and unilets are very different.