Sanctions are for not turning up to sign on
No, they're not. You can be sanctioned for all sorts of reasons.
If you don't spend enough time looking for job - you have to job search for 35hrs p/wk, minimum.
If you don't hand out enough CVs - each week you're supposed to contact a set number of employers on speculation.
If the Universal Jobmatch computer matches you to a job and you don't apply for it - even though the computer makes errors like matching you to jobs you where you don't meet the criteria or are completely unsuitable for you.
If you're ill - you get 'signed off' on the basis that, if you were offered a job that day, you wouldn't be able to work and your claim then has to go to a decision maker who will decide if you'll be sanctioned and how long for.
If you're late to your appointment - even in reasonable circumstances like there being a transport strike or adverse weather, even when it's their fault because you were on time but they forgot to mark down that you'd arrived. It goes to a decision maker who, again, can decide to sanction you.
And yes, if you miss your appointment for any reason including having a heart attack a few hours before your appointment and missing it because you're in HDU. They can, and do, sanction you for that.