Declare Expressions used to concatenate the first name and last name. It's like a declarative rules, In declarative rules we have forward chaining whenever inputs change.
Decision table is when you want to evaluate the single property against some conditions you will be going for decision table. It needs to be referred in the activity by using property map decision table or or by using decision shape.
Decision tree is like a loop ,if the particular condition is true then it will enter into the loop, Otherwise it will not move inside the loop.