Grey stripe/box on forms

[MissVinegar]

Hi,
Maybe it's silly but I have this narrow grey stripe with an arrow pointing right on the left hand side of the detail section (in form view) on every form I've made that I've never seen on template forms. It doesn't bother me that much but it messes the design a bit. Could someone tell me what this is?

[aytee111]

Form properties, Record Selectors = No

[MissVinegar]

Oh, thank you so much!