Pipe lining cannot fix a belly in a sewer line. The liner cures to the shape of the pipe it sits in, so the dip stays and water still settles there. A shallow sag in an otherwise sound line can often be lined anyway. A deep belly that holds standing water needs that section dug up and relaid.
A belly is a slope problem, not a pipe-wall problem. Lining rebuilds the wall: it seals cracks and separated joints so roots and soil stop getting in. It does not lift a sunken section. On a mild sag that still drains, sealing the pipe can be the right call, with the low spot kept clear by periodic cleaning. A severe belly is different. Waste settles in it, clogs keep coming back, and a liner would only give you a smoother pipe holding the same pool of water.
In North Texas the usual cause is the soil. Expansive clay shrinks in dry summers and swells after rain, and that movement can drop a section of pipe into a low spot while it pulls other joints apart. Many lines end up with both problems. The common fix is split: excavate only the bellied section, relay it on a firm base at the fall a sewer line needs, then use sewer pipe lining on the cracked remainder.
One honest limit: there is no single fixed cutoff between a minor and a severe belly. It is judged on camera by how deep the standing water sits in the pipe and how long the sag runs. A sewer camera inspection shows both, and the footage is worth seeing before anyone quotes lining on a line that holds water.