In the Roman Catholic church, a transitional deacon is a man under a vow of celibacy who has been ordained a deacon and who intends to become a priest. For several centuries, all Roman Catholic deacons were in this transitional stage between layman and priest. The role of permanent deacon, usually a married man under a vow never to remarry after ordination if his wife predeceases him, was revived after the Second Vatican Council in the 1960s.
It is beyond the purview of this analysis to discuss in detail the erroneous nature of Pope Paul's decision, which has by and large resulted in the creation of a constituency group dedicated to the protection of its own interests and to de-emphasizing the role of the priest in the life of a parish.
His service as a transitionaldeacon prepares him more fully for his ordination as a priest, and it equips him to serve as a deacon in a Solemn High Mass (just as his service as a subdeacon prepares him for that role even after his ordination to the priesthood).
The permanent diaconate was superceded by the transitionaldiaconate in order to emphasize, correctly, the need of the Church for priests, not those who desire some of the privileges and powers of holy orders while remaining free to pursue their interests in the secular world.
The diaconate has often been regarded as a transitional order, pertaining typically to the first year of ordained ministry, and as an apprenticeship for the priesthood.
In general, where it is permissible for the president to delegate specifically diaconal functions to another minister (including Communion by Extension), these should be entrusted to the deacon where one is available.
A priest is also a deacon, and it will be appropriate for a deacon to give special attention to the specifically diaconal ministries to which the priest, by virtue of his or her priestly responsibilities, has a need to share.