Just as an afterthought, in case I haven't explained myself very well.
ESO - means eye(s) turn IN (varying degrees)
EXO - means eye(s) turn OUTWARDS (again, varying degrees)
With 'phoria', you have the ability to control, or suppress, this squint, be it eso, or exo
With 'tropia', the eye stays turned in/out; - you cannot control it yourself.
With the type you CAN control, some people find, for example, they have no difficulty with their eyes during the day, but can find the control 'harder' to establish when they are tired: This is only one example, there are many more
FD