By convention, anything following a double-underscore now represents a literal subscript in a variable name.
For example, a variable with the name a__b represented in Maple Input ("1-D") notation appears as 'a' with a subscript of 'b' in Standard Math ("2-D") notation, but the 'b' is not interpreted as an index of any kind. Previously, such a variable would have a 1-D representation of something like `#msub(mi("a"),mi("b"))`.
By default, Maple will highlight atomic variables using a different color, making it easier to discriminate between index- and literal-style subscripts when examining a worksheet. The appearance is user-modifiable using Maple's style management tools or can be toggled on or off using the View - Atomic Variables menu item.