Is there any difference between "have been used" and "have been so far used"?

Question

Is there any difference between "have been used" and "have been so far used"? e.g., in:

we first review and categorize the frameworks that have been so far employed to evaluate PIR methods.

Answer 1

Yes, there is a difference. The expression "[things] have been [done]" implicitly means "to this date". Adding "so far" might seem redundant, but it's not! It adds an additional implication that what has taken place up to that point may not continue to take place, or if it does continue, may be adjusted, embellished or taken to a different level at some point in the future.

So far – ODO
1 To a certain limited extent:
"jabs and pills can protect you only so far"

2 (Of a trend that seems likely to continue) up to this time:
"diplomatic activity so far has failed"

In your example, it seems likely (from the limited context) that "so far" is being used to flag a change of direction. The addition of the phrase gives the reader advance notice that you are about to leave the familiar evaluation framework behind and embark on a new stage or level of the process.