Portuguese term
Australia
Works in: Portuguese to English
poderes e atribuições de assinatura isoladamente
"A administracao da sociedade caberá ao socio XXXX, poderes e atribuições de assinatura isoladamente, autorizado o uso do nome empresarial, vedado, no entanto em atividades estranhas ao interesse social..."
Proposed translations
United States
BA in Transl., MA in ESL, 15 yrs of exp.
Native in: Portuguese 
Works in: English to Portuguese, Portuguese to English, Spanish to Portuguese, and 2 more.
(who shall have) the power and responsibility of signing individually
"A administração da sociedade caberá ao socio XXXX, com poderes e atribuições de assinar isoladamente..."
In translation, I would add a relative clause and get rid of the preposition. I would also use the two nouns in their singular form since we are talking about one type of power and one type of responsibility.
atribuição: Responsabilidade específica de cargo ou função, ou de quem os exerce; COMPETÊNCIA; OBRIGAÇÃO - http://www.aulete.com.br/atribuição
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Note added at 10 hrs (2019-04-13 12:54:41 GMT)
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One more correction: "sócio"
Brazil
IT, Games, Linguistics and Marketing
Native in: Portuguese (Variant: Brazilian)
Works in: English to Portuguese, Portuguese to English, Spanish to Portuguese, and 1 more.
individual executing powers and assignments
https://books.google.com.br/books?id=lMQYAAAAYAAJ&pg=PA308&l...
https://books.google.com.br/books?id=oo5WnBpAKDEC&pg=PA242&l...
Talvez "individual signatures to powers and assignments"?
Brazil
32 years engineering/law/contracts
Native in: English (Variant: US) , Portuguese (Variant: Brazilian) 
Works in: English to Portuguese, Portuguese to English
Brazil
Business, Corporate & Legal Translations
Native in: Portuguese (Variant: Brazilian)
Works in: English to Portuguese, Portuguese to English
(who shall be) authorized and in charge of signing separately/individually
United States
25 + yrs. Business experience PORT- ENG
Native in: Portuguese , English
Works in: Portuguese to English, English to Portuguese
(who shall be) the sole authorised signatory
http://www.businessdictionary.com/definition/authorized-sign...
Officer or representative vested (explicitly, implicitly, or through conduct) with the powers to commit the authorizing organization to a binding agreement. Also called signing officer. See also signing authority.
IMHO, this definition sums up the boilerplate wording in PT
"poderes e atribuições de assinatura"
https://smartcounsel.gtlaw.com.au/hrf_faq/company-by-authori...
(see example for Australia here......"click here")
https://tinyurl.com/yxebba2y
Austria
Retd.T&I, Barrister & Austro-Advokat
Native in: English (Variants: Scottish, British)
Works in: Spanish to English, English to Spanish, French to English, and 5 more.
with all manner of power and authority to sign on his own
Power and authorities in the plural sounds unnatural in English, so needs to reworded with a different slant.
(1) In favour of a person dealing with a company in good faith, the power of the directors to bind the company, or authorise others to do so, is deemed to be free of any limitation under the company’s constitution.
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