Click this link and many of your questions on operating a digital tacho will be answered. Or to put it another way RTFM. 
It’s a pdf file so you will need Acrobat Reader to read it.
cant you remember when you was a kid coffee, why, but why does that do that, but why, 
That manual should answer most of those whys. It is for only one make of digi tacho but the basics will be the same so it should be useful.
I’ll give it til Wednesday before someone posts a question on the main forum about them .
As long as Wednesday Rob? I’m thinking tomorrow.