Fund Manager normally automatically calculates distributions per share for you. This dialog is presented in the case where Fund Manager is unable to perform this calculation. This will happen when you own 0 shares for the date(s) associated with the distribution you are entering.
Fund Manager uses the distribution per share figure in calculating various performance figures such as %Gain-F, or when plotting any of the Price + Dist. graph types. Using this dialog allows you to specify what distribution per share value Fund Manager will use in these calculations.
If at a later time you record a transaction which causes you to own shares on the date(s) associated with this distribution then Fund Manager will automatically begin calculating this figure for you. Until then, Fund Manager will use the value you specify in this dialog.
Situations which may cause this dialog to be presented include if you are just monitoring an investment that you don't own any shares in. In this case, record a distributed distribution for a value of $0, and then specify the distribution per share in this dialog. This enables accurate performance calculations of investments you don't own.
Another situation which could cause this dialog to be presented is if you are recording transactions out of chronological order. For instance, if you are entering many transactions, and record a distribution that occurred after some purchases, which you just haven't entered yet. In this case, you can enter any value here. Once you record your purchases, Fund Manager will automatically calculate the distribution per share for you.