Update Item to Revision vs Revert to Revision

To understand how the state of your working copy is different in both scenarios, you must understand the concept of the BASE revision:

BASE

The revision number of an item in a working copy. If the item has been locally modified, this refers to the way the item appears without those local modifications.

Your working copy contains a snapshot of each file (hidden in a .svn folder) in this BASE revision, meaning as it was when last retrieved from the repository. This explains why working copies take 2x the space and how it is possible that you can examine and even revert local modifications without a network connection.

Update item to Revision changes this base revision, making BASE out of date. When you try to commit local modifications, SVN will notice that your BASE does not match the repository HEAD. The commit will be refused until you do an update (and possibly a merge) to fix this.

Revert to revision does not change BASE. It is conceptually almost the same as manually editing the file to match an earlier revision.

Update to revision will only update files of your workingcopy to your choosen revision. But you cannot continue to work on this revision, as SVN will complain that your workingcopy is out of date.

revert to this revision will undo all changes in your working copy which were made after the selected revision (in your example rev. 96,97,98,99,100) Your working copy is now in modified state.

The file content of both scenarions is same, however in first case you have an unmodified working copy and you cannot commit your changes(as your workingcopy is not pointing to HEAD rev 100) in second case you have a modified working copy pointing to head and you can continue to work and commit