Level Restrictions
The servers do enforce level restrictions, so a host cannot summon co-op helpers or be invaded by enemy pvp players who are not close to the same level. The level restrictions for Co-op or for Black Phantom invasion are the same. Add 10 to 10% of your current Soul level to find the range (above or below your SL) within summoning or being summoned by other player(s).
Soul level <10 10 50 100 280 712
Restriction ± 10 ± 11 ± 15 ± 20 ± 38 ~ ± 81
Note two players' level restrictions will not be the same if they are not the same level, but both must meet their own restriction test. A level 150 can co-op with levels 125 to 175, but a SLvl 125 player will have a restriction range of only ±22, so they could not play because the lower level players upper limit is only 147. So the effective lower level range for a 150 player is 128.