Our current (undocumented) guidance is

• \tcode{foo} is used for stuff that may occur in source code, given a suitable environment.
• \placeholder is used for code meta-variables, e.g. exposition-only types and concepts
• Other meta-variables and indexing use math mode: $\tcode{T}_i$

First, we should document this policy. Second, we should also apply it to the atomics meta-variables when constructing all sorts of partial orders throughout [basic] and [atomics]. That is, these should change from \placeholder to $math mode$.