This discussion paper lays groundwork for possible future proposals to accommodate use cases for opus, serial, and thematic index numbers not currently acknowledged in RDA. It discusses creating a new element supertype, Expression: numeric designation of musical expression, and three element subtypes as narrower elements of Expression: identifier for expression. This would follow the model of the work element Work: numeric designation of musical work and its element subtypes. Other options are to add only Expression: numeric designation of musical expression with specific options for different types of identifiers, or simply add additional Condition/Option sets to Expression: identifier for expression. Alternatively, if such refinements are deemed out of scope, they may be left to community practice. This paper also discusses problems related to lingering language and unexamined assumptions from AACR2 in the element subtypes for Work: numeric designation of musical work.