hArLtRoN, 'offset' is a parameter in 'create partition' that makes the partition start after the offset. E.g. in your case, it would be 'offset=100000000' because the first partition is 100GB.
On the other hand, it says in the diskpart description:" If no offset is given, the partition is placed in the first disk extent that is large enough to hold it." But I am not 100% sure what that means. It could be that the partition is then put into the next available freespace - or not.
That's why I suggested to go the safe route and define that partition later with Disk Management.