INFLD marks the position and size of the input field in a variety of ways. In text (non-GUI) mode, the standard marker is an underline character. If the prehistoric compatibility mode is selected (|p), then the field markers will be dots. You can make the markers invisible with type I.  

When a GUI-enabled terminal emulation is used (one that ends in G, such as PCTDVG or AM62CG or AM75G), by default INFLD will use a sunken panel effect to identify the position and size of the field. This effect only works well (visually) when the screen background color is somewhere in the gray or pastel zone; otherwise it may be hard to see. You can disable the sunken panel effect and force it to use the underlines instead with type |_ (vertical bar, underline).

In full GUI mode (a GUI-enabled terminal emulation and type |G specified), the field becomes a Windows control (typically an edit control, but other possibilities are up/down controls, combo boxes, checkboxes, and radio buttons), and no other field markers are needed.