"Be matter of" is a phrase that means to be of concern or importance. There are a plethora of synonyms that can be used in place of this phrase, including "be of consequence," "be significant," "be relevant," "be at stake," "be important," and "be of interest." Each one of these synonyms carries a slightly different connotation, but they all convey the same basic message. These phrases are often used in formal writing or in professional settings to discuss important issues that require attention, such as business deals or legal matters. It is important to choose the right synonym for the situation in order to accurately communicate the level of concern and importance.