The Per-Line Tax Configuration tab is only displayed for tax classes that qualify for per-line taxes, which are local and VoIP. By default, taxes are applied per line and no configuration is necessary.
Configuration is needed for multi-channel lines, for example, taxing a trunk line as if it were multiple lines.
A PRI, for instance, has a 23/1 split: 23 voice and 1 data.
By taxing per channel, you can set the voice and data for outbound or inbound for each channel.
The taxes are then computed by the tax engine on a per channel basis versus for the PRI as a single line.
How it is taxed is based on the configuration in the tax database and the service tax configuration.
Taxes in the tax database allow for channel taxing, but only if the service per line tax is set up accordingly.
The per line tax class dropdown shows what items are supported for channel taxing (PBX Trunk, PRI, Centrex and other multi-line channel).