In the 1917 Code of Canon Law, there were sins a simple priest could not absolve and persons he cannot absolve. These were called "reserved sins."
The 1917 Code allowed as well as a diocesan bishop in his own See to reserve absolution of certain sins. In the 1983 Code, however, there are no longer any such reserved sins but a number of reserved cases which carry certain canonical penalties (i.e. censures,, excommunications, suspensions, etc) still remain.
In six catagories of sins, removal of the penalties is reserved to the Holy See:
1. Apostasy, heresy, schism
2. Violation of the sacred species
3. Physical attack on the pope
4. Absolution of an accomplice in a sin against 6th commandment
5. Unauthorized ordination of a bishop; this penalty is incurred by both the
ordaining bishop and the bishop who is ordained
6. Direct violation by a confessor of the seal of confession
In nine catagories, it is the bishop:
1. Procuring an abortion (but not murder of an adult nor performing an abortion)*
2. Mechanically recording or divulging by a technical instrument in the
communications media what was said by either a confessor or a penitent
in a sacramental confession, whether performed by oneself or by another
3. Physical attack on a bishop
4. Pretended celebration of the Eucharist by a non-priest
5.. Attempt to impart sacramental absolution or hear confession by one who cannot
do so validly
6. False accusation (denunciation) of a priest of having committed the crime of
solicitation in the confessional
7. Attempted marriage, even civil, by a religious in perpetual vows
8. Cleric who attempts marriage, even merely a civil marriage
9. Cleric who is ordained by a bishop who does not have legitimate dimissorial
letters
* all US bishops have delegated to confessors the power to remove the penalties when granting absolution,.