Mutualism is an ecological interaction between two species where both species benefit from the interaction. The interaction may be obligative or faculative and diffuse or pairwise.
Obliagate vs Faculative
Obligate is the restriction to only one niche strategy.
Faculative is non-obligation to use a typical niche, lifestyle, or behavior, but may use it under some conditions such as faculative mycorrhizal plants that associate with mycorrhizal fungi when soil nutrients are limited.
Diffuse vs Pairwise
Diffuse is a mutualism strategy in which species cooperates with an array of potential partners.
Pairwise is a mutualism strategy in which species can only cooperate with each other, and not with any other partners.