Publicly available, public domain, and open source
Information that is available to the public is excluded from export controls. However, there are differences between how the ITAR and EAR treat this exclusion. They share the idea that publicly available information should not be subject to export controls. They differ in how the information can be made publicly available. Additionally, the ITAR refers to information "in the public domain" while the EAR refer to "publicly available" information.
The ITAR states that information in the public domain that is published and that is generally accessible or available to the public is excluded from control as ITAR technical data. See ITAR paragraph 120.11 for details on the definition of published and generally accessible or available to the public.
The EAR excludes from its control publicly available technology and software, except software classified under ECCN 5D002 on the Commerce Control List (certain encryption software), that are already published or will be published. See EAR part 734.3(b) for details on the definition of publicly available and published.
Information in the scope of the ITAR can be made publicly available only by the means described in section 120.11, or with the explicit authorization of the State Department. Information in the scope of the EAR can be made publicly available by a person who is not restricted from doing so (e.g., restricted due to a non-disclosure agreement or a distribution restriction).
Another meaning of "public domain" is a work whose intellectual property rights (copyright, patent, trademark) have either expired or never existed. This is different than export controls meanings. For example, a piece of software can be copyright (not in the public domain from an IP perspective) but published and generally accessible (in the public domain from an export control perspective).
"Open source" isn't public domain in the IP sense because it typically has an owner who provides it, often free, under a license that grants a wide range of rights and obligations to the user. But it can be in the public domain from the export control perspective.