Deif is the leader of Hamas's military wing, while Sinwar is another senior member of Hamas. Kuntar is a Hezbollah operative who participated in a Nahariya terror attack that killed five Israelis, was imprisoned, and later released before returning to Lebanon. Mushtaha was also released in the same prisoner exchange, and is a member of the political branch of Hamas.
The designation means that all property subject to US jurisdiction in which the individuals have any interest is blocked and US persons are prohibited from engaging in transactions with them or to their benefit.
The timing of the announcement – during the week that Congress is expected to vote on the nuclear deal with Iran – may be intended to emphasize for all parties in the Middle East that Barack Obama's administration will intensify its activities to prevent the spread of Iranian-sponsored terrorism and the transfer of weapons to Hezbollah and Hamas.
By adding Kuntar to the United States' international terrorist list as a person who helps terrorists and who is himself a terrorist, the administration states that Kuntar is a US target and it is illegal to help him in any way.
Regarding Kuntar, who was released from an Israeli prison in 2008, the State Department sad, that following his return to Lebanon, he joined Hezbollah and "has since emerged as one of the group’s most visible and popular spokesmen. Since Kuntar’s return, he has also played an operational role, with the assistance of Iran and Syria, in building up Hezbollah’s terrorist infrastructure in the Golan Heights."
Sinwar and Mushtaha were released in the prisoner-exchange deal that released captive Israeli soldier Gilad Shalit.
Sinwar was given the prisoners' portfolio for the Hamas government in Gaza, and Mushtaha is a member of the political branch of the organization and a founding member of its military wing.