It sounds like the restriction may be that you cannot act out-of-uniform to “kill, injure, or capture” an adversary, but can do so to gather information and the like.
However, if caught out of uniform, someone also loses Geneva Convention protections for prisoners of war.
disguising a warship to appear to be a neutral merchant vessel, or a merchant vessel on the side of one’s opponent, has traditionally been considered a legitimate ruse de guerre, provided the belligerent raises their own flag to break the deception before firing their guns. This was called sailing under false colors. Both sides during the world wars used this tactic, most famously the Royal Navy’s Q ships. The German raider Kormoran used this tactic against the superior HMAS Sydney, disguising herself as the Dutch merchant vessel Straat Malakka prior to their mutually destructive engagement.
It is prohibited to kill, injure or capture an adversary by resort to perfidy. Acts inviting the confidence of an adversary to lead him to believe that he is entitled to, or is obliged to accord, protection under the rules of international law applicable in armed conflict, with intent to betray that confidence, shall constitute perfidy. The following acts are examples of perfidy:
(a) The feigning of an intent to negotiate under a flag of truce or of a surrender;
(b) The feigning of an incapacitation by wounds or sickness;
(c) The feigning of civilian, non-combatant status; and
(d) The feigning of protected status by the use of signs, emblems or uniforms of the United Nations or of neutral or other States not Parties to the conflict.
Ruses of war are not prohibited. Such ruses are acts that are intended to mislead an adversary or to induce him to act recklessly but which infringe no rule of international law applicable in armed conflict and which are not perfidious because they do not invite the confidence of an adversary with respect to protection under that law. The following are examples of such ruses: the use of camouflage, decoys, mock operations and misinformation.
The principle of distinction is one of the core principles of IHL, and the requirement to operate while wearing a uniform or a distinctive sign is generally considered one of the fundamental criteria for attaining combatant status. However, after examining the relevant sources such as the Geneva Conventions and their additional protocols, we have concluded that the mere operation of combatants without uniforms does not violate IHL. Nevertheless, since that requirement is also one of the basic conditions for conferring POW status upon combatants, the failure to wear a distinctive sign while conducting a military attack could result in the loss of the right to POW status in international armed conflicts, and thus the loss of combatant immunity.
After analyzing the prohibitions and their elements, we have reached the following conclusions:
(a) Customary nature of the prohibition - Treacherous and perfidious killing or wounding, originating in article 23 of the 1907 Hague Regulations and article 37 of Additional Protocol I, are customary prohibitions under IHL and are applicable to all parties in international and non-international armed conflicts. The prohibition against capturing a person while resorting to perfidious ways, appears only in article 37 of Additional Protocol I and does not constitute a customary prohibition, and therefore, it applies only to states party to Additional Protocol I.
(b) The mere use of undercover units does not constitute a violation of the prohibition of perfidy and treachery. Only undercover operations aimed at the killing or wounding of an adversary are prohibited as perfidious and treacherous.
(c) The violation of the prohibition of perfidy and treachery becomes applicable from the point that the undercover forces are de facto visible to the adversary during the deployment to attack, and therefore trust can be established.
(d) Undercover operations aimed at the gathering of information are not prohibited as perfidious or treacherous acts and could be considered acts of espionage.
EDIT: The above distinction pointing out that the prohibition on use of perfidy specifically to capture is not customary international law – that is, binding on everyone, regardless of what treaties they are party to – and only affects states party to Additional Protocol I shouldn’t apply to Russia, as Russia is a state party to Protocol I.
So I’d expect Russia can probably have soldiers legally act out of uniform, but if captured, they will not receive the protections the Geneva Convention affords people with POW status, and if they act out of uniform to kill, injure, or capture Ukrainians, this will be a war crime violating one or both of customary international law or the Geneva Conventions.
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EDIT4: Misread Wikipedia; it looked like WP had said that Russia had revoked ratification of Protocol I in 2019, but in fact, they revoked an associated statement attached to the ratification. So Protocol I’s prohibition on acting out of uniform to capture Ukrainians does indeed apply in Russia’s case.