During this phase, you may destroy any of your own ships.  A self-destructed ship is immediately destroyed, and cannot fire any guns that turn, although it can be the target of enemy fire (since all combat happens simultaneously).  Any cargo ships loaded onto the destroyed ship are also destroyed; further, self-destructing a ship which is loaded or boarded onto a carrier ship causes that carrier (and any other cargo) to be destroyed, as well.

A ship which is self-destructed cannot be salvaged by other empires.  Additionally, for every five tonnes (or fraction thereof) massed by the destroyed ships and cargo, every other unloaded ship in the same sector receives one DP of collateral damage (i.e., self-destruction of less than five tonnes worth of ships will cause one DP of collateral damage to other ships in the same sector, self-destructiong six to ten tonnes will cause two DP of collateral damage, and so on).  All self-destructed tonnage in a sector is totaled together, regardless of owner, in order to compute the collateral damage to other ships in the sector.  A loaded or boarded ship does not receive collateral damage, but it will be destroyed if its carrier is destroyed due to collateral damage.

Note that only ships that are explicitly self-destructed are exempt from salvage; any ship destroyed by collateral damage can be salvaged.

You cannot self-destruct your starbase, nor can your starbase be destroyed as the result of self-destructed cargo; however, a starbase will be damaged normally by collateral damage inflicted by other self-destructing ships.
 
Abandoned Empires: Abandoned non-missiles will be self-destructed at foreign worlds if it appears that there are enough foreign guns present to destroy those ships, and if it appears that the abandoned ships will not also be able to destroy all of the foreign ships.  If it appears that there are enough foreign guns present to destroy only some of the abandoned ships, then only those that would be destroyed by the foreign guns (ordering the abandoned ships by the criteria in the Fire Guns phase) will be self-destructed.  The remainder will be moved out-sector.