The Session save option allows you to save much of the information about the loaded tables in your current TOPCAT session. Unlike the Current Table or Multiple Tables options, the whole of each loaded table, along with other information about your use of it, is saved, rather than just the Apparent Table. The saved items include:
The list of loaded tables is shown, with a column of checkboxes on the left. By default these are selected, but you can select or unselect them by clicking. When the save is performed, only those tables with the checkbox selected will be saved.
The tables are saved as a multi-table fits-plus (recommended) or VOTable file. This is a normal multi-table file in that any FITS or VOTable reader can read the tables it contains, but it contains some specialised metadata encoding information of special significance to TOPCAT, marking it as a saved session. The upshot is that if you save a file in this way and then load it back into TOPCAT, the tables it loads will appear in very much the same state as when you saved them, in terms of defined and current subsets, row order, visible and invisible columns, and so on. If you process it with some application other than TOPCAT, it will look quite like the table you saved, but with some additional data and metadata.
Note however that the reloaded state is not identical to that before saving. Only state belonging to tables is saved, so that for instance the state of Plot and Match windows will not be restored, and table activation actions are not saved either. It is possible that some of this may be changed in future releases.
The session save format has changed at TOPCAT version 4.5. Up to v4.4, columns and row subsets that had been defined algebraically were saved as fixed values, so the expressions were lost and it was no longer possible to edit them following a session save/load. At v4.5, the expressions themselves are saved and reloaded, which means the state is preserved better after a session save/load cycle, and may also result in smaller saved files. Post-v4.5 versions of TOPCAT should be able to load sessions saved by pre-v4.5 versions, but not vice versa. This also means that if you try to load a v4.5-format session into a non-TOPCAT reader, the algebraically-defined columns will not appear, but session-saving is not recommended for use with other table applications in any case.