button.
The Score is always computed from white's perspective, so 100% means all White wins and 0% means all Black
wins. Games with no result do not contribute to the percentage drawn, but as half-won/half-loss.
The
Scid's Tree search is fast because of a novel idea - we search for games that do not reach this position!
One can open a database and then select Windows--> Tree Window, use the Control-T short-cut, or Open Base as Tree from the file menu. This last method means games in one database can be examined via the tree from a different database.
The Tree Window shows statistics about the next move, as calulated from all games in the current base.
Extended info is shown by the button.
The Score is always computed from white's perspective, so 100% means all White wins and 0% means all Black
wins. Games with no result do not contribute to the percentage drawn, but as half-won/half-loss.
Clicking on a move adds that move to the game. Middle-click shows/hides the button bar.
The moves in the tree window can be sorted by Move (alphabetically), ECO code, Frequency, or Score.
The Adjust-Filter checkbox alters the gamelist/filter so that only games matching the current position are displayed. Its behaviour is a little complicated. Deselecting Adjust-Filter will set the regular filter to the current (adjusted) filter. But closing the Tree, will restore the regular filter to how it was when Adjust-Filter was selected.
The 
button in the Tree window, shows games matching the current position.
Normally, the highest rated games are displayed first, but by changing the 'Sort' button, games will be shown in their normal database order. (The default for this feature can be altered in the Tree options menu).
You can also restrict the list to show games with a particular result, and alter the displayed columns by right-clicking the column titles.
Tree Masks provide additional information beyond pure statistical data, and can be imagined as a transparent layer above the current tree that holds additional data. For more info see here.
Each tree window is associated with a specific base, that is, if several bases are opened simultaneously, several tree windows may exist. If Tree-> Options-> Lock is enabled, closing the tree window will also close the database associated with this specific tree. Additionally, this also closes associated graph or best games windows.
Opening a base as tree from the file menu will lock a database by default. <\i>
The Training button 
enables a basic Opening Trainer feature.
Moves in the tree are hidden, trial mode is engaged, and
Scid replies to in-tree player moves with one of it's own, according to database weight.
So a move existing in 80% of database games will be played 80% of the time.
There are addition functions associated with Tree Masks, but these are untested by this author.
The Tree Cache is not as important as previously, as Scid now has an Interuptible Tree feature.
Scid maintains an in-memory cache of tree search results. This allows the Tree Window to update much faster when the current position is in the cache. Configuration of this feature is via the Tree Window menus.
This cache can be written to disk for future use with the Save Cache command, which writes the cache to a file with the suffix .stc, in the same directory as the database. One may also select Auto-Save Cache to automatically save the cache when the base is closed; though this may not work when Scid is quit before closing a base.
The Fill Cache File command fills the cache with data for around 100 of the most common openings.
The maximum size of the Cache is configurable, and defaults to 1000 lines. The cache file (.stc) is completely disposable. One may remove it without affecting the database, and in fact it is discarded by Scid whenever an action occurs that could leave it out of date - for example, adding or replacing a game, sorting the database, or decreasing the cache size.
Alternatively, one may fill the cache with the contents of selected games or a single game, including all variations. Generally, the cache will not be large enough for numerous games. In this case, less common lines will be discarded for those more common. This feature is useful if you have one, or more, repertoire bases that can serve as input.
Tree Refresh can be sped-up if the database is sorted
by ECO code and then compacted (see the maintenance
window). Once this is done (which can take some time),
turn on the option Fast Mode. Tree Window refresh will be 20 times faster on average
at the cost of some inaccuracies; games not in current filter will not be taken
into account. Also, Fast mode does not update the Tree Cache, nor
consider move transpositions. To allow for this there is a third Tree mode.
Fast and Slow Mode initially gives one a preview of statistics, then performs a slow update afterwards.
For more info see Pascal's comment's
https://sourceforge.net/p/scid/mailman/scid-users/thread/48103F94.1010408%40free.fr/