DOCSIS 3.0 (an old standard from 2006) indicates the fastest throughput available:
- with that device AND
- IF you had an unlimited Internet Plan AND
- you had perfect service in your area.
The manufacturer's web page for the product also confirms a maximum speed of 343Mbps when it gets to us 8 download channels.
The Internet Plan Speed is what you chose when you signed up for service with that provider (Comcast? Your linked photo #1 suggests Comcast may be your provider). Different providers, different areas; different maximum speeds.
You might have signed up for 10Mbps, 100Mbps, 1000Mbps... if those speeds are available in your area. Most cable companies do not offer you everything the (2006 design DOCSIS 3.0) hardware can do (much less DOCSIS 3.1), because they haven't invested in the fiber optic infrastructure to let everyone run at maximum speed.
Your table showing Extreme/Ultimate will cost more than the Economy/Performance plan, but delivers nearly 3x the throughput IF there are 24 download and 8 upload channels (yeah, the 6MHz wide channels also used for cable TV) available where you are. Sometimes there are channels open, sometimes, there are not, because it Costs Big Money to buy right-of-way privileges, dig ditches, lay cable, document everything, and make the connections.
Comcast, in theory, has at least one home in 80% of its markets on DOCSIS 3.1, a significantly better standard. Doesn't mean they have it in your market, doesn't mean that even if they do have it in your market, that the cable infrastructure where you are allows DOCSIS 3.1 speeds.
There's a very nice paper from MIT which, although 23 years old (back when ISDN was Hot Stuff) which introduces some basic concepts.