Cross-reference lines are used in MR and CT series to identify the relative anatomical position of a collection of cross-sectional images. Scout or reference lines indicate the position of each cross-sectional image.
Firstly, you have to display several different series in Split Screen mode. Cross-reference lines are shown on inactive views and indicate the position of the image on active view. 
Reference lines are 3 types:

current reference line is shown in red. Also, slice thickness is displayed with dotted lines.

boundary reference lines are shown in green. They depict the first and last image in a series.

intermediate lines are shown in yellow and depict reference lines on all images in a series.

You can switch on or off current, boundary and intermediate lines in the Measure and annotate menu.

There are settings for Cross-reference lines in the Options dialog(Tools/Options). 

switch on/off visibility

change colors

change opacity

show slice number