Yellow line

[Jimurray]

T7, Lucroy's fly ball passes out of the defined playing field in flight but hits below a yellow line, live ball, not a HR. No complaints but MLB stadiums complicate things.

[Gfoley4]

how was it out of the defined playing field if it was below the defined yellow line

[Jimurray]

Take a look at the crooked boundary fence/wall below the yellow line that extends perpendicularly from the field. 

[Rich Ives]

12 hours ago, Jimurray said:

Take a look at the crooked boundary fence/wall below the yellow line that extends perpendicularly from the field. 

The yellow line is the boundary marker. Period. 

(A ball hitting the yellow line is in play BTW)