Where can I get information on criteria to determine what is an MQL, and eventually a SAL and SQL? We are a tech managed services provider, with a wide portfolio and, as expected, a complex buyer behavior. I cannot depend on online conversion as a metric, since most marketing and sales engagement takes place "offline". My challenge is to set clear engagement for a lead generation  specialist in my team, and eventual handover to the sales team.

Having a filter it's important, because as you said, most conversations and lead generations take place online, some people in your industry are only trying to take a look at the product and not buy, so being more specific to who your target audience is (professionals in the field) can help to filter out the ones who aren't your priority and that can take action. 

Here's are a couple of helpful link: 

https://mountain.com/blog/mql-vs-sql-vs-sal-whats-the-difference-why-do-they-matter/

https://6sense.com/blog/the-perfect-guide-to-lead-qualification/

A couple of suggestions:
-if you're in SaaS suggest exploring Product Led Growth (PLG) principles and 'Product Qualified Lead (PQL). See: https://productled.com/blog/product-qualified-leads
- use a quantification mechanism (eg. lead scoring) to nurture contacts to become a MQL. For example, I would advise against simply calling all sign ups to an event a MQL without qualification.
- filter out contacts with / without buying intent eg. in the contact for ask for role, and if a student most likely they are looking for education and do not plan to buy soon
- map contact to your ICP (ideal customer profile) then segment nurture flows (eg. email) based on persona (eg. CIO vs hands on operations)
- definitely hold a regular meeting with Sales to review the contacts, status, and their acceptance

Hope that helps.