Buoyancy Force

Hello everyone,

I know that there was a discussion about buoyancy forces acting on a rigid body but i spent some time for solving this problem and I will appreciate if you help me about my situation.

First of all I need to model and graph all the forces acting on an arbitrary body in a closed cube of stationary water . The density of my particle will be close to the density of water.
From that i have to make an animation for observing the movement of my arbitrary body.

I followed instruction for the example terminal velocity of falling sand grain. But i have some question which i dont understand in that example's instruction pdf.
1) What is Xdott, Xdot?? and where do you define those?
2) What does reacf() operator stand for? Does it include all acting forces like for example in vertical movement drag and buoyancy forces?

Thank you