SOAP WSS, what is Offset and Length?

Question

SOAP

I find these parameters in the examples, but I can’t find descriptions. What do these parameters define?
For example

<ns19:Offset>0</ns19:Offset>
<ns19:Length>24</ns19:Length>
<ns19:Nonce>UPbRPlbcnbr0Ff1COI/iXThY</ns19:Nonce>

and

<ns19:Offset>0</ns19:Offset>
<ns19:Length>32</ns19:Length>
<ns19:Nonce>gxe1lXgmL/tYVJRWJXO5F2Hq</ns19:Nonce>