Which ideology was advocated by the socialists which distinguished them both from the Congress as well as from the Communists?

Democratic Socialism.

The Socialist Party originated during the mass movement stage of the Indian National Congress in the pre-independence era.
In 1934, a group of young leaders formed the Congress Socialist Party (CSP) within the Congress, seeking a more radical and egalitarian approach.
In 1948, Congress amended its constitution, not allowing its members to have dual membership, forcing the Socialists to form a separate Socialist Party.
The Socialists differentiated themselves from both the Congress and the Communists by advocating democratic socialism.
The Socialists faced a dilemma when Congress declared its goal as a socialist pattern of society in 1955 (Avadi Resolution), making it difficult to present themselves as a distinct alternative.