Two parsimony values are displayed in the top area of the ARB_PARSIMONY main window:
the current parsimony value (representing the costs of the current topology)
the optimal parsimony value (=the best parsimony value seen so far)
When adding species to the tree, the ´Parsimony value´ will normally raise. That means the previously seen 'optimal parsimony value' may be too low now and reaching a better value by ´Tree optimization´ may be impossible.
Use this function to reset the optimal parsimony value to the current parsimony value.