OCCISCO identifies, according to the major categories in the International Standard Classification of Occupations (ISCO) scheme for 1988, a person’s primary occupation. Whereas INDGEN is an industry classification; where industry refers to the activity or product of the establishment or sector in which a person worked. Which you should use depends on the purpose of your analysis (i.e. are you interested in occupational information or industry information). For example, a person may work in the agricultural industry, but that does not necessarily mean they are a farmer.
We also have several other work related variables available for international samples that you may wish to explore.