What are the differences between $BL$ and $Bx$ in ARM assembly instructions?

$Bx$ is a directive, is a basic but commonly used means of implementing subroutine invocation. BL is a return from the subroutine.

$BL$ is a directive, is a basic but commonly used means of implementing program. $Bx$ is commonly used means of implementing process.

$BL$ is a directive, is a basic but commonly used means of implementing subroutine. $Bx$ is a basic but commonly used means of implementing functions.

$BL$ is a directive, is a basic but commonly used means for calls subroutine invocation. $Bx$ is used to return from the subroutine.