This will happen Alex :

The Process flow will work like normal. I did have popup for each row on but that shouldn't matter.

That look almost as what I need, but you only iterate the Task-step. Not the entire processflow. 

My processflow; 

We first have a task to validate if Account match. If not, a message with the option to quit of continue anyway. If continue, the next task will do the math, set the invoices, book the purchases and update the Purchase_line as ‘Executed’. (That’s where the filter comes in).

So, what I want is that the WHOLE flow is done, per selected grid line. check the account, push (if needed) the message, do the math… NEXT line. 

Is this possible?

That is not possible as it can't be guaranteed that you only do actions that don't change the focus of the screen or switch the focus to a different row.

It's probably better to do this all in the Task stored procedure instead of doing this in a process flow. That does mean you lose the ability to use message options; due to using tsf_send_message in code.

Are the message options very important?

Ah, this was just what I was afraid of. I’ve made the whole thing, but now it’s useless. 

Well, the message is not that important, but when I trigger the message in my task-code I can only send something like “Suppliers do not match” as a notification. The task will still be executed. If the user does not want to continue, it is not possible to stop. A flow with a yes/no option makes this so much better for the user. 

You can make use of Multirow task execution | Thinkwise Community (thinkwisesoftware.com). That way it will be considered one task execution for all rows you selected. With some process flow magic, you could loop the message + true-task execution] per row in the 'Multirow task execution parameter’. 

The process flow starts like normal but this time with the multirow task execution parameter, that parameter will be captured in a process variable. Next-up is a decision node that uses that process variable to decide with which row to continue and to erase that row from the tmultirow task execution parameter]. So then you have the parameter with all rows to be handled and you have subtracted the row you will be using in the flow.

After the decision node, show the message and then based on the response, either execute the task and return to the decision node, or directly go back to the decision node to get the next row to handle. 

This does do the trick for you