SINEplayer handles Legato in a special way.
Most sample libraries, when they say "legato" actually mean "sustains with connected notes". Generally, those transitions between notes will be recorded ("true legato"), but they are only used for sustained articulations.
Orchestral Tools sees "legato" not as a patch type, but as a property that you can apply to a number of articulations, not just sustains. When an OT instrument has "legato", that means that we have recorded one or more legato styles, which SINEplayer will let you use for any articulation(s) it makes sense for. That will be all the "longs" like sustains, tremolo, etc.., but also some articulation somewhat in the middle like marcato. You can not use legato transitions for short articulations like staccato, simply because it makes no sense to do so.
Default Sustains + LEG
By default, every instrument in SINEplayer will have a special articulation called "Sustains + LEG" displayed in a separate colour in its articulation list.
This articulation is a normal Sustains articulation (functionally identical to the Sustains below it), but will have Legato enabled by default. We include this because it is a common use case and show it in a different colour to show that it is not a separate articulation. You can load this Sustains + LEG like any other articulation, you can load multiple copies of it with different settings, etc...
Setting up Legato
To use Legato on an articulation where it is not enabled yet, make sure that articulation is selected. Click on it so it shows a dot next to the name, and the Articulation Options in the lower right will show its properties. In the Leg tab, you can then set up the transition speed zones and styles the way you need them.
Note that the number of possible speed zones depends on the number of available legato styles.
Click the Legato button, either in the Leg tab or the Main tab, to enable/disable Legato. When Legato is enabled, articulations will be monophonic.
Note that you can only use Legato on articulations where it actually makes sense.