Packed/Unpacked array - what object/like 0 or 1?/how to check/

Question

In Mathematica, there is a concept called packed array.

In[1]:= ClearAll[A, B, L1, L2, L3, L4];
A = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 
   19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30};
B = Range[30];
L1 = Developer`FromPackedArray[A];
L2 = Developer`ToPackedArray[A];
L3 = Developer`FromPackedArray[B];
L4 = Developer`ToPackedArray[B];
In[2]:= A === B === L1 === L2 === L3 === L4
Out[2]= True
In[3]:= {ByteCount[A], ByteCount[B], ByteCount[L1], ByteCount[L2], 
 ByteCount[L3], ByteCount[L4]}
Out[3]= {768, 360, 768, 360, 768, 360}

1. For what Mathematica objects, does the concept of 'packed/unpacked' exist? (only for List?)

2. We can define compression method (7z/zip/tar/...) or compression level (store/very fast/fast/normal...) for archive utility like 7-zip. Likewise, are there various degrees/methods/kinds of packing in Mathematica? Or it is just 'packed or unpacked' like '0 or 1'.

3. How can we check/verify a Mathematica object is packed/unpacked ? (+ inspect the degree/methods/kinds)

I have more questions :

4. Does Developer`FromPackedArray do nothing for unpacked array ?
5. Does Developer`ToPackedArray do nothing for packed array ?
6. Are there any option for Developer`FromPackedArray/Developer`ToPackedArray ?
7. Are there any other function that can change the state of packed/unpacked (or degrees/methods/kinds) ?