Glossary entry (derived from question below)
Portuguese term or phrase:
tratamento sigiloso
English translation:
Confidential treatment
Portuguese term
United States
Experienced, skilled, reliable
Native in: English 
Works in: French to English, Catalan to English, Spanish to English, and 1 more.
tratamento sigiloso
Proposed translations
Confidential treatment
Private and not for disclosure
secret/ top secret
Cheers, John.
Professional in Oil & Gas Industry
Native in: Portuguese
Works in: English to Portuguese, Portuguese, Portuguese to English
In confidentiality
Portugal
over40 yrs professional accredited work
Native in: English , Portuguese
Works in: English to Portuguese, Portuguese to English
confidentiality guaranteed
echoecho (X)
Translator of Cultures DE-ES/EN
Works in: Spanish to English, English to Spanish, German to Spanish, and 4 more.
Confidential/clasified information
secret=secreto
top secret/secreto exclusivo?
my friends in Brazil
Portugal
Very accurate, I never miss deadlines.
Native in: Portuguese
Works in: English to Portuguese, French to Portuguese
strictly confidential
echoecho (X)
Translator of Cultures DE-ES/EN
Works in: Spanish to English, English to Spanish, German to Spanish, and 4 more.
Confidential/clasified information
secret=secreto
top secret/altamente secreto?
only inside the government
my friends in Brazil
Brazil
Public/Sworn Translator - Brazil
Native in: Portuguese 
Works in: English to Portuguese, Portuguese to English
This information should be handled in strictest confidence.
strict confidentiality
If this is the context, then I would say that this would be one option.
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