The 2 90 rule or 2/20 rule for credit cards is an American Express rule for credit cards that states that you can only be approved for 2 new credit cards within a 90 day time period.
And if you apply for a third American Express credit card within the 90 day time period window, that third American Express credit card application will be denied automatically.
The 9/20 rule for credit cards applies to credit cards specifically and not to charge cards or hybrid cards, like The Platinum Card or the Amex Gold Card.
However this 2/90 rule for credit cards is specific to American Express and not used by other credit card issuers that don't issue American Express cards.
The 2/90 rule for credit cards is a rule that was implemented by American Express and not a universal rule across all of the credit card issuers so you can apply for other credit cards through other credit card companies during this 90 day time period.
You're only limited to two credit card approvals through American Express, but you can apply for other credit cards such as Visa, Mastercard, Discover etc.
The third application for an American Express will be automatically denied within the 90 day period but other credit cards such as Discover, Mastercard or Visa credit cards may be approved.
Also American Express also has other application rules for credit cards, like the 1 in 5 rule that limits you to one American Express credit card approval within every 5 days.