proprietorship firm

No there is no Disadvantage of it you can put the board of the firm.

Any unincorporated business, such as a sole proprietorship, can readily start doing business at the residential address.

As long as the location is appropriate for the specific firm, it can be registered

Registration of a firm from residential house shall be considered as "commercial" property for the all purposes . 

If the firm is registered then you can display the signboard, but would depend upon the types of activities undertaken, if that is creating any nuisance the neighbors will certainly sue for that.

Is it considered as a non residential even if its partially used for business purpose with proprietorship board put outside residential property

For all purposes, the registration of a business from a residential dwelling is deemed "commercial" property. I recommend you to join this website to get more athentic information.

The business is done online clients are not visiting only board is hung outside residential building no nuisance is created whether neighbour or bbmp can create prblm

If the property exclusively belongs to you then you can put up the board. The local municipality may have  a higher rate of tax for commercial use of the premises. They may demand higher tax. There may be fire and other regulations for commercial premises. You may have to comply with them. If you have salary paid employees, there may be laws regarding that also such as a Shops and Establishments Act. If your premises are part of a residential building or residential complex, then the permission of the Society or Assossiation may also be required.