There are several views as to the meaning of the "lawful sanctions" exception.
The sanctions must be of the kind widely accepted as legitimate
Special Rapporteur on torture, Nigel S. Rodley, was tasked by the U.N. Commission on Human Rights to report on the practice of corporal punishment through the lens of various human rights instruments, including the UNCAT. He reported (E/CN.4/1997/7):
the "lawful sanctions" exclusion must necessarily refer to those sanctions that constitute practices widely accepted as legitimate by the international community, such as deprivation of liberty through imprisonment, which is common to almost all penal systems. Deprivation of liberty, however unpleasant, as long as it comports with basic internationally accepted standards... is no doubt a lawful sanction. By contrast, the Special Rapporteur cannot accept the notion that the administration of such punishments as stoning to death, floggin and amputation -- acts which would unquestionably be unlawful in, say, the context of custodial interrogation -- can be deemed lawful simply because the punishment has been authorized in a procedurally legitimate manner, i.e. through the sanction of legislation, administrative rules or judicial order. To accept this view would be to accept that any physical punishment,
no matter how torturous and cruel, can be considered lawful, as long as the
punishment had been duly promulgated under the domestic law of a State.
The Committee Against Torture has said all corporal punishment violates the Convention Against Torture
See Concluding Observations on South Africa, UN Doc. CAT/C/ZAF/CO/1, 2006, §25:
The State party should ensure that legislation banning corporal punishment is strictly implemented, in particular in schools and other welfare institutions for children, and establish a monitoring mechanism for such facilities.
The "lawful sanctions" exception does not expand what is allowed under the Convention Against Torture because article 16 is more restrictive anyway
Article 16 forbids "other acts of cruel, inhuman or degrading treatment or punishment which do not amount to torture" anyway, with no exception for "lawful sanctions." Article 1's "lawful sanctions" exception, when read in conjunction with Article 16, may be meaningless. See C.E.F. Coracini, "The Lawful Sanctions Clause in the State Reporting Procedure Before the Committee Against Torture" (2006) 24 Netherlands Quarterly of Human Rights 305, p. 317:
If the sole arguable application of the 'lawful sanctions' clause is to prevent the treatment of prisoners, consistent with international standards, from falling within the threshold of the concept of torture, and considering that the treatment of prisoners in accordance with international law should not violate the prohibition of torture, the clause is potentially a useless provision, which at the present time, ought not to be taken into account by interpreters.
Another view is that this is not clearly resolved
Organisation Mondiale Contre la Torture (OMCT) says in their handbook:
the issue is not resolved, and it may be that this exception exempts even the cruellest treatment from classification as "torture" if such treatment is authorised by domestic law. [citing Johan D. van der Vyver, "Torture as a Crime Under International Law" (2003) 67 Albany Law Review 427, p. 432]
The United States, in the context of its extradition regulations made to comply with the UNCAT, understands "lawful sanctions" to include (see 22 C.F.R. § 95.1):
judicially imposed sanctions and other enforcement actions authorized by law, provided that such sanctions or actions were not adopted in order to defeat the object and purpose of the Convention to prohibit torture