The U21 players could be first teamers too then.
So it reads only states the 6 being u23's.
This is from the EFL site on rules
For the EFL Clubs, a registered player is one who would be registered and eligible to participate in a match in the League Competition for that EFL Club commencing at the same time and on the same date as the Match in accordance with the provisions of the Regulations.
Each EFL Club shall play its full available strength in and during all Matches. The League will from time to time issue a policy as to what constitutes 'full available strength'. Any Club failing to meet this requirement will be required to pay a fine of up to £5,000.
For the Invited Clubs, a registered player is one who would be registered and eligible to participate in a Premier League and/or Premier League 2 match commencing at the same time and on the same date as the Match in accordance with the provisions of the relevant Premier League rules.
Each Invited Club must ensure that in each Match at least 6 of the starting eleven players named on the Team Sheet are players who were aged under 21 as at the 30th June in the year in which the Season concerned commenced.
The restrictions set out in Regulation 52.4 shall apply (number of loan Players on a Team Sheet) to all Clubs as if set out herein.
Players registered on a temporary basis will be eligible to play unless the loaning Club (or club) expressly indicates it does not consent as part of the applicable loan agreement.
No Player may play for more than one Club in the Competition in any one Season.
Read more at
http://www.efl.com/global/trophyrules.aspx#aS71l3q0a2eD6gQ8.99