These are like identifiers, but may also include periods, plus, and minus characters. A normal identifier will map to a file variable when it is not defined as another expression. To force variables that look like integers or floating point numbers to be interpreted as variable names, or to force variable names which are defined by another expression to map to a variable in a file, they should be enclosed with < and >, the left and right carats/angle brackets. Note that quotation marks will cause them to be interpreted as string constants, not variable names. In addition, variables in files may be in directories within a file, so they may include slashes in a path when in angle brackets.
Examples: density <pressure> <a.001> <a.002> <domain1/density>