Question about the tangential behavior of ABAQUS

Hello everyone, I am using ABAQUS to study the tangential behavior (friction) at the interface between rock-machine. I know tangential behaivour simualtion is key for my model, I read the manual, but still have some puzzels, hoping you could help me, much appreciated in advance!

1. what's the difference among mechanical constraint formulation of penalty contact method (another is kinetic), friction formulation of penalty for tangential behavior, and  constraint enforcement method of penalty for normal behavior. the explanation in manual seems ambiguous for me.

2. what's the difference between frictional constraints enforced with a stiffness (penalty method), and softened tangential behavior? I think they are same approach, but they are listed separately in the manual below.

3. the manual said that default penalty stiffness for frictional constraints is chosen automatically by Abaqus/Explicit; and If tangential softening is specified, the penalty stiffness will be equal to the value specified. but when I tried, if I didn't specify the elastic slip stiffness below (= no slip), it seems that Abaqus/Explicit doesn't enforce default penalty stiffness and it acted like infinite elastic slip stiffness, and the contacts keep sticking along the whole process (sense like rough tangential behavior).

4. the section below mentioned the softened contact for  three times, I am not sure are they  all associated with tangential behavior (penalty with elastic slip), or normal behavior as well? and what does it mean by hard contact, is that hard contact for normal over-closure?