Sentient..by my take anything that may be able to suffer..usually that means anything with some type of aversion or attachment response, something that eats and avoids in some manner discomfort.
Trees for instance are not thought of as being suffering things.
Maybe they are but most say no.
As a aside I do this though it makes not much sense(I also sing dharma songs to cows and things
)..so consider the source.
I care not if I don't intend to kill but kill(the things suffer)....so weeding out a garden or some such driving killing some bugs...it is said that if a being hears the name of buddha ratnaketu upon their dying they will enter as consequence upon rebirth this place of purity a pureland. So if I do such things even unintentionally I will say this name so they may have a fortunate rebirth.
If I kill a larger animal I will do powah if I may.....squirrels and all sorts of things are ending up in purelands and it is me sending them there...
I wonder if they wonder how come that place is getting so crowded
Anyway it is the least I can do...consequence upon me as it is not intended..... my consequence is always present and undeniable.... it is my sorrow for creating their fearful death. That there is no consequence is in error to my opinion even if not consciously intended things.
I intend its continuance by not becoming enlightened and thusly continuing to participate in this place where things may kill and be killed....it is all intentional to my opinion.....we must work to not participate in this fashion and become enlightened.
Otherwise we intend it but in subtle fashion.
"This order considers that progress can be achieved more rapidly during a single month of self-transformation through terrifying conditions in rough terrain and in "the abode of harmful forces" than through meditating for a period of three years in towns and monasteries"....Takpo Tashi Namgyal.