In the regulations shared with the franchises on Wednesday, the BCCI has said categorically, "Impact Player can only be Indian player unless there are less than 4 overseas players in the Playing XI."
"Only an overseas player who is part of 4 substitutes named in the team sheet can be used as an Impact Player. If a team introduces an overseas player as an Impact Player in a match, in no circumstance can a 5th overseas player take the field," the three-page note states.
The bowling team can also get an Impact Player during the fall of the wicket, but that Impact Player will not be permitted to bowl the remaining balls in that over if the wicket has fallen mid over.
The BCCI says an Impact Player cannot act as a captain. The particular explanation goes like this:
"In case of Impact Player used by a team, and if an injury occurs, the same rule will apply as they currently do under playing condition (refer Rule 24. Fielder's Absence; Substitutes). If umpires are satisfied that a fielder has been injured or becomes ill during the match, a substitute fielder is allowed to field in place of an injured player. The substitute shall not bowl or act as captain. Penalty time for the batting and fielding teams will apply to the player off the field for the substitute as per the playing conditions."
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