The general difference is the filtering on filters tab is based on values in the connected (parent/remote) table, while the show is based on filtering data based on either the form or filter by related records.
You can see very detailed docs here with the 3 different types of filtering:
But I’ll try and clarify here as well.
Filter Records:
In your Add Activity table if you wanted to only show Projects that have a Status (field) set to “Active” you would do that in the Filters tab. Since this is filtering based on fields in the parent table. In other words, filter based on values in the remote/parent table.
Show Filters:
Show filters are based on restricting dropdown connected values based on other form or page values.
A Basic example is Dependent Dropdown. A more complex example, is to filter based on the page you’re currently on.
Here’s I hope a relatable example for that.
If you’re a Company with Users (Each user belongs to a Company). Now, suppose you’re inside the details page and you’d like to add a new Job and assign it to a user. But since you’re on a company called “Google” you should only see the Users who belong to this page’s Company. Not Users that work form Microsoft.
There is one more filter which we call limit, to limit records that this user connects to. For this lets assume we have 3 tables. Users, Companies and Jobs. Each User belongs to a Company.
So now when we add a new job and assign it to a company, we only want to limit the companies to records Logged In User is connected to. Again, this we can’t use the filters tab, since each user connects to a company, not the other way around. If the Company connected to Users then you would use the Filters tab.
Rules of thumb:
If you’re filtering by an actual field in the remote table, use the filters tab. Otherwise, you must use the other options.