Free slack and total slack:

The total arc slack is refers for the maximum amount of slack time available. If you begins an activity later than the TE of the node at its tail, by an amount equivalent to the total arc slack, this Pushes the node at head of the arc to its latest time. This has consequence downstream: activity later then the head node events may now not be able to use their total stack. it always ok to use up the free arc slack because this has no downstream consequence. However if you wish you push an activity later by any amount of time between the free slack and total slack .you must be careful then there are no negative downstream consequence. Alternatively, you can schedule activity B-C to start any time between TE(B) and  TE(B)+free   slack B-C without fear 

Although you want to schedule activity B-C to begin sometime between TE(B)+free  slack B-C and TE(B) +(total slack B-C) then there may be downstream consequence. Because node c will be push later then TE(c).the below table summarised free slack and total slack for all non critical activity. Because both of the slack is always zero for critical activities.