ANSWER 1:

the various Applications of Stack in Data Structure:

• Evaluation of Arithmetic Expressions
• Backtracking
• Delimiter Checking
• Reverse a Data
• Processing Function Calls

answer 2: 

A queue may have a limited space depending on the implementation. We must implement check conditions to see if we are not adding or deleting elements more than it can maximum support.

The underflow condition checks if there exists any item before popping from the queue. An empty one cannot be dequeued further.

if(front == rear)
// underflow condition

The overflow condition checks if the queue is full (or more memory is available) before enqueueing any element. This prevents any error if more space cannot be allocated for the next item.

if(rear == SIZE-1)
// overflow condition