UEFA has detailed the reasons behind four controversial VAR incidents in this week's Champions League matches.
UEFA has defended the decision to award Manchester United a late penalty in their Champions League victory over Paris Saint-Germain on Wednesday.
A 3-1 win at Parc des Princes sent United into the quarter-finals on away goals following a 3-3 aggregate draw.
The crucial goal came from the penalty spot in second-half injury time, with Marcus Rashford firing home after Presnel Kimpembe had been penalised for handball following a lengthy VAR review.
The decision was slammed by injured PSG star Neymar, who was watching from the stands, and has been much debated in the days since, with some arguing Kimpembe could do little to prevent blocking Diogo Dalot's shot with his arm.
UEFA has now explained the process that led to referee Damir Skomina awarding the spot-kick.
"The VAR, after checking various different angles available to him, recommended to the referee an on-field review following the penalty area incident," European football's governing body said.
"Given that the referee did not recognise the incident clearly during live play (referred to as serious missed incident in the VAR protocol) an on-field review was conducted.
"Following the on-field review, the referee confirmed that the distance that the ball travelled was not short and the impact could therefore not be unexpected. The defender’s arm was not close to the body, which made the defender’s body bigger, thus resulting in the ball being stopped from travelling in the direction of the goal. The referee, therefore, awarded a penalty kick."
It was one of four key decisions in last-16 second-leg matches this week, with a questionable call occurring in the build-up to an Ajax goal against Real Madrid and two penalty claims in Porto's defeat of Roma.
At the Santiago Bernabeu, Madrid were left furious that Dusan Tadic's third goal in a 4-1 win for Ajax was allowed to stand after Noussair Mazraoui had attempted to stop the ball going out of play in the build-up.
"There was no conclusive evidence that the ball would have been entirely out of play from all video angles and images that were carefully analysed by the VAR," UEFA said.
"The assistant referee, who was perfectly positioned, had adjudged that the whole ball had not fully crossed the touchline. No on-field review was therefore required. Consequently, the referee was right not to intervene and to allow the goal."
In Portugal, Fernando was awarded a penalty from which Alex Telles scored the decisive goal in extra time to send Porto into the last eight at Roma's expense.
VAR was used for that decision, but referee Cuneyt Cakir did not check the pitch-side monitor five minutes later, when Patrik Schick felt he had been tripped in the Porto box.
"The VAR, after checking the offside line - which confirmed the attacker to be onside - asked the referee if he had seen the holding offence committed by the AS Roma defender," UEFA's explanation about the first incident read.
"The referee confirmed he was unaware of any holding during live play and he asked for the images to be prepared to allow him to conduct an on-field review (serious missed incident). The review convinced the referee that a penalty kick should be awarded for a holding offence."
It added on the Schick claims: "The referee was close to the action and had himself seen the potential incident in live play and judged that there was no foul.
"The referee nevertheless decided to delay the restart of play, to give more time to the VAR to review the different camera angles available. A VAR check was conducted, and the various images were studied carefully by the VAR, who did not find any clear evidence.
"The referee was then informed by the VAR that following the check no clear and obvious error had occurred and that there was no ground for a VAR intervention and an on-field review."