When it comes to the transfer window, what is the Financial fair play rule? How does it impact the spending limit for the clubs?
Here is a link that should answer your question. It has a chart showing the different financial information regarding the teams spending caps.
The Financial Fair Play rule was put in place by the UEFA in 2009 with the purpose of ensuring that clubs in Europe did not over spend and risk the longevity of their team to try and be successful. The above link contains a chart that notes the specific numbers for the Champions, Barclay and other European leagues. It essentially limits how much they can over spend on players so that it keeps all the teams at a level playing field financially. It forces clubs to be more responsible with who they trade for.
Basically, this rule was put in practice to stop unfair advantage for the "rich teams"(Chelsea FC, Manchester City, PSG). According to this rule, there is now a spending limit that the teams have to abide by, such that their net spending (total loss - total profit) cannot be more than a particular amount in one season.
The profit here includes all money earned by the club by means of players sold, franchise money, equipment's sold (jerseys etc).
PSG failed to abide by this rule, so this year they have been imposed a fine of £49m and a reduced squad(2 players less) for Champions league.