In a 在zài sentence, the subject is definite. The word or phrase indicating the location can be either definite or indefinite.
English: A book is on the table.
zhuō shàng yǒu yī běn shū 。
(Incorrect: 一本书在桌上。 (一本书 is indefinite; it cannot be the subject.))
(This sentence is not to indicate ‘location’, but to indicate ‘existence’. Therefore, it is not a 在 sentence in Chinese.)
wǒ de chē zài nà kē shù xià 。
My car is under that tree.
wǒ de chē zài yī kē shù xià 。
My car is under a tree (a certain tree).
English: A car is under the tree.
nà kē shù xià yǒu yī liàng chē 。
(我的车 and 那棵树 are both definite; 一棵树 and 一辆车 are both indefinite.)
 Yip, P. C., Rimmington, D., Xiaoming, Z., & Henson, R. (2009). Basic Chinese: a grammar and workbook. Taylor & Francis.