question in the blood vessel tutorial

In the attached tutorial, on page 15, the force (boundary load) in the x,y,z directions is specified with a negative sign. I am wondering if someone can explain what the negative sign signifies. I am asking because when I look under the fluid properties, the stress vectors are defined as spf.T_stressx , spf.T_stressy and spf.T_stressz and not -spf.T_stressx,-spf.T_stressy and -spf.T_stressz.

Thanks for the help.

Sirisha