"Data caps or limits on usage" refers to a monthly usage cap - many carriers limit consumers to some quantity of gigabytes downloaded or uploaded per month. Sonic's services are unlimited for household use.
What you're referring to is tiered speeds, and this is a bit more confusing because where we deliver on our own technology platform (Fusion, and Fusion Fiber), we run the link at whatever the maximum technology speed is. There is an X2 variant on Fusion, with two lines for $20/mo additional, but in this case too, both lines run at maximum speed. So someone might have X1 at 10Mbps and X2 at 20Mbps, while another user has X1 at 50Mbps - just because they're closer to our network equipment.
But on the resold access via AT&T's network, there are three versions, an X1, X2 and X3, and while often the X2 uses two lines and the X1 uses one, it's not a one to one relationship, due to the way that they structure both the technology and pricing. It sounds like you've got Fusion IP Broadband X1, at 20Mbps. The next level for that product is X2, which would deliver 50Mbps, and that'd generally be done by delivering a second line, and the cost is again $20/mo additional. Making it even more confusing, where AT&T has fiber, the speeds are different for the X1/X2 and X3 products.
But to make this all simpler: at a particular location there is a single set of products, with specific speeds and capabilities. So at your site if Fusion IP Broadband X1 is the product, you'll see 20Mbps on AT&T's network. An X2 product might also be offered. The fact that another service like Fusion (on copper, with VDSL at XMbps) or Fusion Fiber (on Sonic fiber at 1000Mbps) might be available across town isn't really relevant - at least until we get that built in your location. And then you'd see that offered as the upgrade path!
The best spot for current customers to easily see their site's specific technology, price, and upgrade availability is in the Member Tools, in the Internet Connections section of the menu. Here's a direct link: https://members.sonic.net/connections/serviceupdate/