Initial cell – random valid cell, starting point of the ship construction. Use getInit(grid).
Ship body – only the straight part of the ship. For example, bodyLength of regular 4 cells ship is 4, but LShaped 4 cells ship has bodyLength equals to 3, because its last cell is curved. This is the tail.
Body direction – determines the building direction of the body. Use buildBody(initCell, bodyLength, grid).direction.
Ship tail – curved part of the ship. Only LShaped ships have it. Tail length is always equals to 1. Use buildTail(lastCell, bodyDirection, grid).
Tail direction – yes, tail also has its bulding direction. The direction of the tail is always fixed ( because 'L' letter can't be mirrored) and depends on the direction of the built body. For example, if we were building ship body up from the initial cell, the tail direction will always be equal to left.
Last cell – the last cell of the building body. Being used for building a tail.
Note, that built ship has not only its own coordinates, but also the coordinates of the surrounding/occupied cells. Why? According to the rules of Battleship game ships must not touch each other and must have at least one cell between them. Use buildShip(grid, type).occupiedCoords or its modules – buildBody(initCell, bodyLength, grid).occupiedCoords and buildTail(lastCell, bodyDirection, grid).occupiedCoords.