"constitution" is the process of appointing a lawyer to act in a case - including notification of appointment to the court and to the other parties.

The barrister who actually pleads may be different - especially in the provinces when a Parisian lawyer is drafted in to plead but does not practice locally and cannot act directly before the court.

As for the wording - I consider "briefed and pleading" to be adequate although briefed does not perfectly equate to "constitué".

Alternatively "Having as Counsel, entering appearance and pleading" gives the meaning but is more cumbersome. I tend to go for this option if one or more of the parties have two sets of lawyers, since the difference between the "procedural" and "pleading" jobs is important.