There are three possible decisions for a Social Security disability claim: fully favorable, unfavorable, and partially favorable. A partially favorable decision grants part of a disability claim.
Occasionally, a partially favorable decision makes a determination that you were disabled for a period of time, but are no longer disabled and not entitled to ongoing benefits. This is essentially a closed period.
Usually however, a partially favorable decision finds that you are currently disabled, but decides that you became disabled under SSA's rules sometime after your alleged onset date. You will receive ongoing disability benefits. However, a later onset date means a lesser amount of retroactive benefits than would have been paid with the original alleged onset date
This can be a big deal. I recently saw a partially favorable decision that used an onset date 17 months later than the alleged onset date. So while the client will receive ongoing benefits, 17 months of past benefits were left on the table. With a PIA of $800, that is $13,600.
The decision whether or not to appeal a partially favorable determination presents a dillemna for the claimant. If the appeal is successful, you will be found disabled as of your alleged onset date, and will recoup those lost benefits.
However, by appealing a partially favorable determination, you appeal the entire decision. This means that on appeal you could be found "not disabled," and would lose your entitlement to ongoing benefits. This does not usually happen, but it is a risk that must be considered.