Linking Members To Dynamics Contacts
When you set up Dynamics in Administration Panel, you can tick 'Create Contact on New Member' tickbox or leave it unticked.
If you tick this tickbox, whenever new Member registers, Externable will create a coresponding contact in Dynamics and link Externable Member account with a contact. If you decide not use this feature, Externable Members will not be linked to Dynamics contacts, so you will need to link them manually in Externable admin panel - otherwise you wouldn't be able to use member contact guid value in your FetchXML/SQL queries in Externable.
You don't have to do it, however if you do it, Externable comes with handy features to inject linked Dynamics contact guid into your queries. This way you can present the user Dynamics data, which user is allowed to see. It is highly recommended to rely on those parameters given by Externable as they are resolved server side by authentication mechanisms and give you confidence that current user really is linked to certain contact.