Text Alpha
txt-alpha
The txt-alpha parameter fades a text layer. It accepts a value from 0 to 100 and defaults to 100, fully opaque.
The fade applies to the whole layer at once — the fill, the outline, the shadow and the background box all fade together, keeping their relationship to one another. This is what you want for a subtle text watermark, where fading each piece separately would make the outline stand out against a faded fill.
To make only the box translucent and leave the text solid, give txt-bg an 8-digit color with its own alpha instead, such as txt-bg=80263E65.