UEFA announced on Tuesday it had decided to remove the "cup-tied" rule from all European competitions.

This means that, for the first time, a player could play for two different clubs in the group stage and the knockout rounds of the Champions League, or the Europa League, in the same season. Players could switch between competitions in the past, but not stay in the same one with a second team.

What prompted UEFA to scrap the 'cup-tied' rule?

It follows pressure from a number of high-profile clubs and players, most notably this season from Barcelona and Arsenal.
Barca signed Philippe Coutinho from Liverpool in January for a fee that could rise to €160 million, but as he had already played for the Premier League club in the group stage he was therefore ineligible for his new team in the Champions League until next season.
Arsenal fell foul of a little-known rule. They signed Pierre-Emerick Aubameyang from Borussia Dortmund, who had played for BVB in the Champions League. The Gunners were in the Europa League, but as Dortmund finished third in their UCL group and therefore dropped into the UEL, Aubameyang was ineligible for Arsenal. If both clubs are in the same competition a player cannot play for the second club.

Why was the "cup tied" rule in place?

The "cup-tied" rule is enforced in virtually every cup competition in the world. It was designed to prevent wealthy teams signing up good players from other clubs in the same competition to boost their own chances of success (and harm others) in later rounds.
UEFA didn't allow any player to play for a second club whatsoever until 2005-06, when it permitted players to move between competitions only in January. In 2012-13 it then removed any restrictions from players who had played in the qualifying rounds from being registered for another club in the group stage.
Now it has taken the final step and removed the "cup tied" rule completely.

What reason has UEFA given?

In explaining its decision to end the "cup tied" rule, UEFA said: "This is in line with the existing regulatory situation in the different domestic leagues, which does not impose restrictions on the eligibility for competitions of players registered for a new club during the winter transfer window."
However, domestic league competitions have, historically, never had a "cup tied" rule. So the UEFA explanation does not really directly correspond with its cup competition.

When does it come into force?

As of next season, so no change for Coutinho and Aubameyang this season.
How will this affect the January transfer window?
It could have a revolutionary effect. It now means that the richest clubs in Europe can target any player from any other club to bolster their resources for the knockout rounds. It could see far more high-profile, big money moves mid-season when they were previously few and far between.

So the big clubs are happy about this?

Without doubt. It removes any restriction on registering players for the Champions League and the Europa League.

How many new players will clubs be able to register in January?

The three are now all without restrictions. It used to be that only one could have played in the group stage of the other European competition, and only one could have played in the qualifying rounds (from a total of three new registrations).
Can clubs buy from other clubs still in the competition.
Yes, no restrictions.
So a player can now be a winner AND a runner-up in the same season.
Indeed. Let's take Philippe Coutinho as our example. He could have moved from Liverpool to Barcelona in January, with the two clubs then facing each other in the final. This means Coutinho would pick up both a winner's and a runner-up's medal. It may well happen.

Can a player win the UCL and the UEL in the same season?

Yes, but they already could do after the rule was relaxed to allow a player to move between competitions several seasons ago.

Abolishing the rule is harmful

SuperSuave:
Abolishing the rule is harmful

so true, players will be so glad to push for January transfer knowing they will still play in the champions league.

Snails:

so true, players will be so glad to push for January transfer knowing they will still play in the champions league.

especially if it's a bigger team calling

That means Winter's Transfer Window will be a bit busy as Summer among Big Clubs especially when they need certain players to execute their European Competitions.

This cup tie issue has denied some players European winners' medal in the past.

Imagine, Coutinho watching Barca win UCL without playing a role, if they wins, same as Aubumeyang watching Arsenal if they win Europa.

Players like Ronaldo DE Lima etc have not tasted UCL glory in his illustrious career period.
During the Winter Transfer of 2006–07 season Where he moved to AC Milan though Milan won the 2006–07 title, Ronaldo was cup-tied with Real Madrid & ineligible to play for AC. Milan.

In My own opinion, it is a welcome development.