Please could anyone shed light on why my 'set condition' has a blue & yellow shield inside it?

because you have set 'Run as workflow owner'

I haven’t set this to run as workflow owner, it’s just set to run on create?

no, it's jut one of action's common option

Ok that sounds fine, and I’m assuming that it won’t have any detrimental effect on the workflow?

depends what you consider detrimental...

if you check it off the action and all sub-actions it encapsulates will run with credentials of who started workflow. if you start your workflow automatically on item creation then it will be credentials of who created the item.

if you check it on the action and all sub-actions it encapsulates will run with credentials of who published workflow.

so consider what the effect will be in your env.

if it runs on the user who created the item, if there are any actions that require high permissions and the user doesn't have those permissions the workflow may work for some and not for others. Running on workflow owner may allow the workflow to perform actions that require full control.

Let us know if you have any other questions. Marian and Lisa provided some great insights.